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Reference Guide on Engineering
C H A N N I N G R . R O B E RT S O N , J O H N E . M O A L L I ,
A N D D AV I D L . B L A C K
Channing R. Robertson, Ph.D., is Ruth G. and William K. Bowes Professor, School of
Engineering, and Professor, Department of Chemical Engineering, Stanford University, Stan-
ford, California.
John E. Moalli, Sc.D., is Group Vice President & Principal, Exponent, Menlo Park, California.
David L. Black, J.D., is Partner, Perkins Coie, Denver, Colorado.
ConTenTs
I. What Is Engineering? 899
A. Thinking About Engineering and Science, 899
B. Engineering Disciplines and Fields of Practice, 900
C. Cross-Disciplinary Domains, 900
II. How Do Engineers Think? 902
A. Problem Identification, 902
B. Solution Paradigms, 903
III. How Do Engineers Make Things? 904
A. The Design Process—How Engineers Use This Guiding Principle, 904
B. The Design Process—How Engineers Think About Safety and Risk
in Design, 908
1. What is meant by “safe”? 908
2. What is meant by “risk”? 910
3. Risk metric calculation assumptions, 912
4. Risk metric evaluation, 914
5. What is meant by “acceptable risk”? 915
C. The Design Process—Examples in Which This Guiding Principle
Was Not Followed, 920
1. Inadequate response to postmarket problems: Intrauterine
devices (IUD), 920
2. Initial design concept: Toxic waste site, 921
3. Forseeable safety hazards: Air coolers, 922
4. Failure to validate a design: Rubber hose for radiant
heating, 922
5. Proper design—improper assembly: Kansas City Hyatt
Regency Hotel, 923
6. Failure to validate a design: Tacoma Narrows Bridge, 924
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7. Failure to conform to standards and validate a design:
Automotive lift, 924
8. Lack of sufficient information and collective expertise to
consummate a design: Dam collapse, 925
9. Operation outside of design intent and specifications: Space
shuttle Challenger, 926
10. Foreseeable failure and lack of design change in light of field
experience: Air France 4590, 928
IV. Who Is an Engineer? 929
A. Academic Education and Training, 929
B. Experience, 930
C. Licensing, Registration, Certification, and Accreditation, 931
V. Evaluating an Engineer’s Qualifications and Opinions, 932
A. Qualification Issues and the Application of Daubert Standards, 932
B. Information That Engineers Use to Form and Express Their
Opinions, 933
1. Observations, 933
2. Calculations, 936
3. Modeling—mathematical and computational, 936
4. Literature, 938
5. Internal documents, 938
VI. What Are the Types of Issues on Which Engineers May Testify? 939
A. Product Liability, 939
1. Design, 939
2. Manufacturing, 941
3. Warnings, 941
4. Other issues, 942
B. Special Issues Regarding Proof of Product Defect, 943
C. Intellectual Property and Trade Secrets, 945
D. Other Cases, 946
VII. What Are Frequent Recurring Issues in Engineering Testimony? 948
A. Issues Commonly in Dispute, 948
1. Qualifications, 949
2. Standard of care, 949
3. State of the art, 950
4. Best practice, 950
5. Regulations, standards, and codes, 951
6. Other similar incidents, 952
B. Demonstratives and Simulations, 956
VIII. Epilogue, 958
IX. Acknowledgments, 959
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“Scientists investigate that which already is; Engineers create that which has never been.”
Albert Einstein
I. What Is Engineering?
A. Thinking About Engineering and Science
Although this is a reference manual on scientific evidence, the Supreme Court in
Kumho Tire Co., Ltd. v. Carmichael1 extended the Daubert v. Merrell Dow Pharma-
ceuticals, Inc.2 decision on admissibility of scientific evidence to encompass non-
scientific expert testimony as well.3 Put another way, experts not proffered as
“scientists” also are held to the Daubert standard.4 So then we might ask, who are
these nonscience experts and where do they come from? Many emerge from the
realm of engineering and hence the relevance of “engineering” or “technical”
expert testimony to this manual.
The Court’s distinction between these two kinds of expert testimony might
suggest that there is a bright line dividing science and engineering. Indeed, a great
deal has been written and discussed about this matter and arguments made for
why science and engineering are either similar or different. It is a conversation
that resonates among philosophers, historians, “scientists,” “engineers,” politicians,
and lawyers. Apparently even Albert Einstein had a point of view on this issue as
attested to by the above quotation. Perhaps this deceptively attractive dichotomy
is best resolved by recognizing that at the end of the day engineering and science
can be as different as they are alike.
There is no shortage of “sound bites” that attempt to categorize science from
engineering and vice versa. Consider, for instance, the notion that engineering
is nothing more than “applied science.” This is a too often recited, simple and
uninformed view and one that has long been discredited.5 Indeed, it is not the
case that science is only about knowing and experimentation, and that engineer-
ing is only about doing, designing, and building. These are false asymmetries that
defy reality. The reality is that who is in science or who is in engineering or who
is doing science or who is doing engineering are questions to be answered based
on the merit of accomplishments and not on pedigree alone.
1. 526 U.S. (1999).
2. 509 U.S. 579 (1993).
3. See Margaret A. Berger, The Admissibility of Expert Testimony, in this manual.
4. See David Goodstein, How Science Works, in this manual, for a discussion of science and
scientists.
5. Walter G. Vincenti, What Engineers Know and How They Know It (1990).
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B. Engineering Disciplines and Fields of Practice
One can think of engineering in terms of its various disciplines as they relate to the
academic enterprise and the names of departments or degrees with which they are
associated, for instance electrical engineering or chemical engineering. One also
can consider the technological context in which engineering is practiced as in the
case of nanotechnology, aerospace engineering, biotechnology, green buildings,
or clean energy.
In the same sense that some struggle trying to identify the differences and
likenesses between science and engineering, others pursue a different kind of
identity crisis by staking out their turf through title assignment. It is pointless to
list titles of engineering disciplines because such a list would be incomplete and not
stand the test of time as disciplines come and go, merge, diverge, and evolve. Bio-
engineering, biochemical engineering, molecular engineering, nanoengineering,
and biomedical engineering are relative newcomers and have emerged in response
to discoveries in the sciences that underlie biological and physiological processes.
Software engineering and financial engineering are two other examples of disci-
plines that have developed in recent years.
In the end, it is not the names of disciplines that are critical, they being no
more than labels. Names of disciplines are at best imprecise descriptors of the activi-
ties taking place within those disciplines and ought not to be relied on for accurate
characterizations of pursuits that may or may not be occurring within them.
C. Cross-Disciplinary Domains
Whereas engineering disciplines are often associated with their scientific roots (i.e.,
mechanical engineering and physics, electrical engineering and physics, chemical
engineering and chemistry, bioengineering and biology, biomedical engineering
and physiology) some lack this kind of direct association (i.e., aerospace engi-
neering, materials engineering, civil engineering, polymer engineering, marine
engineering). Indeed, there are software engineers, hardware engineers, financial
engineers, and management engineers. There is no shortage of adjectives here.
Nonetheless, these and many other such discipline titles have meant or mean
something to someone, and new ones are emerging all the time as the histori-
cal barriers that once separated and defined the “classic” engineering disciplines
continue to disintegrate and become a thing of the past. No longer can we rely
on discipline names to inform us of specific enterprises and activities. There is,
after all, nothing wrong with this as long as it is recognized that they ought not
be used as reliable descriptors to subsume all possible activities that might be
occurring within a domain. One must reach into a domain and investigate what
kind of engineering is being conducted and resist the temptation to draw conclu-
sions based on name only. Doing otherwise could easily lead to an unreliable and
inaccurate characterization.
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To provide a tangible example, consider cases involving personal injury in
which central questions often revolve around the specifics of how a particular
trauma occurred. In situations where proximate cause is an issue, the trier of
fact can benefit from a thorough understanding of the mechanics that created
an injury. The engineering and scientific communities are increasingly called on
to provide expert testimony that can assist courts and juries in coming to this type
of understanding. What qualifies an individual to offer expert opinions in this area
is often a matter of dispute. As gatekeepers of admission of scientific evidence,
courts are required to evaluate the qualifications of experts offering opinions
regarding the physical mechanics of a particular injury. As pointed out earlier,
however, this gatekeeping function should not rise and fall on whether a person
is referred to or refers to himself or herself as a scientist or engineer.
Specifically, one cross-disciplinary domain deals with the study of injury
mechanics, which spans the interface between mechanics and biology. The tradi-
tional role of the physician is the diagnosis (identification) of injuries and their
treatment, not necessarily a detailed assessment of the physical forces and motions
that created injuries during a specific event. The field of biomechanics (alterna-
tively called biomechanical engineering) involves the application of mechanical
principles to biological systems, and is well suited to answering questions pertain-
ing to injury mechanics. Biomechanical engineers are trained in principles of
mechanics (the branch of physics concerned with how physical bodies respond to
forces and motion), and also have varying degrees of training or experience in the
biological sciences relevant to their particular interest or expertise. This training
or experience can take a variety of forms, including medical or biological course-
work, clinical experience, study of real-world injury data, mechanical testing of
human or animal tissue in the laboratory, studies of human volunteers in non-
injurious environments, or computational modeling of injury-producing events.
Biomechanics by its very nature is diverse and multidisciplinary; therefore
courts may encounter individuals being offered as biomechanical experts with seem-
ingly disparate degrees or credentials. For example, qualified experts may have one
or more advanced degrees in mechanical engineering, bioengineering, or related
engineering fields, the basic sciences or even may have a medical degree. The
court’s role as gatekeeper requires an evaluation of an individual’s specific train-
ing and experience that goes beyond academic degrees. In addition to academic
degrees, practitioners in biomechanics may be further qualified by virtue of labo-
ratory research experience in the testing of biological tissues or human surrogates
(including anthropomorphic test devices, or “crash-test dummies”), experience in
the reconstruction of real-world injury events, or experience in computer model-
ing of human motion or tissue mechanics. A record of technical publications in the
peer-reviewed biomechanical literature will often support these experiences. Such
an expert would rely on medical records to obtain information regarding clinical
diagnoses, and would rely on engineering and physics training to understand the
mechanics of the specific event that created the injuries. A practitioner whose expe-
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rience spans the interface between mechanics (i.e., engineering) and biology (i.e.,
science), considered in the context of the facts of a particular case, can be of signifi-
cant assistance in answering questions pertaining to injury mechanism and causation.
This example illustrates the futility of trying to untangle engineering from
science and vice versa and to the inappropriateness of using semantics, dictionary
definitions, or labels (i.e., degree names) to parse, dissect, or portray the intellec-
tual activities of an expert witness. In the end, it is their background and experi-
ence that are the dominant defining factors—not whether they are a scientist and/
or an engineer and not by the titles they hold.
II. How Do Engineers Think?
A. Problem Identification
Although a somewhat overworked part of our lexicon, it is indeed the case that
“necessity is the mother of invention.” Engineering breeds a culture of techno-
logical responsiveness. All the “science” explaining a solution to a problem need
not be known before an engineer can solve a problem.
Take steam engines, for example. Their history goes back several thousand
years and their utility forged the beginning of the industrial revolution late in the
seventeenth century. It was not until the middle of the nineteenth century that
the science of thermodynamics began to gain a firm ground and offer explana-
tions for the how and why of steam power.6 In this instance, technology came
first—science second. This, of course, is not always the case, but demonstrates
that one does not necessarily precede the other and notions otherwise ought to
be discarded. So here the problem was one of wanting to produce mechanical
motions from a heat source, and engineers designed and built systems that did this
even though the science base was essentially nonexistent.
To reinforce the point that technology can precede science, consider the design
of the shape of aircraft wings. This, of course, was driven by the desire of humans
to fly, a problem already solved in nature since the time of the dinosaurs but one
that had eluded humankind for tens of thousands of years. Practical solutions to this
problem began to emerge with the Wright brothers’ first motive-powered flight and
continued into the twentieth century before the “science” of fluid flow over wing
structures had been fully elucidated. Once that happened, wings could be designed
to reduce drag and increase lift using a set of “first principles” rather than relying
solely on the results of empirical testing in wind tunnels and prototype aircraft.7
6. Pierre Perrot, A to Z of Thermodynamics (1998).
7. The pioneering aerodynamicist Walter Vincenti provides a detailed and fascinating account
of this. See Vincenti, supra note 5, ch. 2; see also John D. Anderson, Ludwig Prandtl’s Boundary Layer,
Physics Today, December 2005, at 42–48.
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So, in short, engineers create, design, and construct because interesting and
challenging problems arise in the course of human events and emergent societal
needs. Whether a science base exists or only partially exists is just one of a myriad
of constraints that shapes the process. Other constraints might include, but are not
limited to, the availability of materials; device shape, size, and/or weight; cost;
demand; efficiency; safety; robustness; and utility. It has been said, and possibly
overstated, but it does make the point, that if engineers waited until scientists
completed their work, they might well still be starting fires with flint stones.
B. Solution Paradigms
So when faced with a vexing and challenging problem, along with its particular or
peculiar constraints, an engineer seeks a path to follow that has a reasonable chance
of leading to a solution. In so doing an engineer must contend with uncertainty
and be comfortable with it. In very few instances will everything be known that
is required to proceed with a project. Assumptions need to be made and here it is
critical that the engineer understand the difference between what is incidental
and what is essential. There are excellent assumptions, good assumptions, fair
assumptions, poor assumptions, and very bad assumptions. Along this spectrum the
engineer must carefully pick and choose to make those assumptions that ensure
the robustness, safety, and utility of a design without undue compromise. This
is the sort of wisdom that comes from experience and is not often well honed in
the novice engineer.
This impreciseness that accompanies uncertainty can be used as a perceived
disadvantage for the engineer in the role of expert witness. Yet it is this very
uncertainty that lies at the heart of technological innovation and is not to be
viewed as so much a weakness as it is a strength. To overcome uncertainty
in design under the burden of constraints is the hallmark of great design, and
although subtle and not always well understood by those who seek precision (i.e.,
why can’t you define your error rate?), this is the way the world works and one
must accept it for what it is. Assumptions and approximations are key elements of
the engineering enterprise and must be regarded as such. And as with all things,
hindsight might suggest that a particular assumption or approximation was not
appropriate. Even so, given what was known, it may well have been the right
thing to do at the time it was made.
In addition to evolving business opportunities and changing financial markets,
technological innovation results from the continuing and many times unexpected
advances in science and technology that occur as time passes. Buildings con-
structed in Los Angeles in the 1940s would never be built there in the same way
now. We have a much better understanding of earthquakes and the forces they
exert on structures now than then. Airbags were not placed in automobiles until
recently because we did not have cost-effective systems and materials in place to
accurately measure deceleration and acceleration forces, trigger explosives, contain
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the explosion, and do this on a timescale that was effective without harming an
occupant more-so than the impending collision. It is unavoidable that as we learn
from new discoveries about the natural world and accumulate more experience
with our designed systems, products, and infrastructure, engineers will be in an
increasingly better place to move forward with improved and new designs. It is
both an evolutionary and a revolutionary process, one that produces both failures
and successes.
III. How Do Engineers Make Things?
A. The Design Process—How Engineers Use This Guiding
Principle
The genesis of nearly every object, thing, or environment conceived by engineers
is the design process. Surprisingly, although products designed using it can be
incredibly complex, the general tenets of the design process are relatively simple,
and are illustrated in Figure 1.
The progression is iterative from two perspectives: (1) Changes in the design
resulting from testing and validation lead to new formulations that are retested.
(2) After the design is complete, performance data from the field can also lead to
design changes.
As a first step, engineers begin with a concept—an idea that addresses a need,
concern, or function desired by society. The concept is refined through research,
appropriate goals and constraints are identified, and one or more prototypes are
constructed. Although confined to a sentence here, this stage can take a significant
amount of time to complete.
In the next phase of the design process, the prototypes are tested and evalu-
ated against the design requirements, and refinements, perhaps even significant
changes, are made. The process is iterative, as faults identified during the testing
phase manifest themselves as changes in the concept, and the testing and evalua-
tion process is restarted after having been reset to a higher point on the learning
curve. As knowledge is gained with each iteration, the design progresses and is
eventually validated, although as alternative solutions are considered, it is pos-
sible that certain undesirable characteristics in the design cannot be completely
mitigated through changes in design and should be guarded against to minimize
their impact on safety or other constraints. A classic example of this step in the
design process is the installation of a protective shield over the blade in a table
saw; although the saw may have the unwanted characteristic of cutting fingers
or arms, the blade clearly cannot be eliminated (designed out) in a functioning
product. As a last resort, anomalies that cannot be designed out or guarded against
can be addressed through warnings. Not every design is amenable to guarding or
warning, but instead the iterative process of testing and prototype revision is relied
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Figure 1. Schematic of the engineering design process.
upon to perfect designs. Indeed, in some instances, an acceptable design solution
cannot be found and the work is abandoned.
The testing process itself can be complex, ranging from simple evaluations
to examine a certain characteristic to multifaceted procedures that evaluate the
prototype in conditions it is anticipated to see in the real world. The latter type
15-1 xed image
of evaluation is often denoted as end-use testing, and is very effective in identify-
ing faults in the prototype. Because many designs cannot be evaluated over their
anticipated life cycle because of time constraints (a product expected to last for
20 years cannot be tested for 20 years in the development process), the testing
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cycle is often accelerated. For example, if it is known that a pressure vessel will
see 50,000 cycles over a 10-year lifetime, those cycles can be performed in sev-
eral months and the resultant effects on vessel performance established. Another
method of accelerating the evaluation cycle involves testing at an elevated temper-
ature and using scientific theory and principles to equate the temperature increase
to a timescale reduction. The efficacy of this approach is highly dependent on
correct execution, but done properly and with appropriate care, it allows product
development to go forward rather than having good or even great designs languish
on the drawing boards because there is no feasible way to validate them under the
exact end-use environment.
Regulations, standards, and guidelines also play an important role in test-
ing of products during the design process. Federal requirements are imposed on
design and testing of aircraft, medical devices, and motor vehicles, for example,
and mostly govern how those products are evaluated by engineers. Standards
organizations such as the American Society for Testing and Materials (ASTM),
the American National Standards Institute (ANSI), and the European Committee
for Standardization (CEN) promulgate test methods and associated performance
requirements for a large number of objects and materials, and are relied on by
engineers as they evaluate their designs. It is critical to understand, however,
that ASTM, ANSI, CEN, and other such national and international standards
organizations describe testing methods that engineers use to obtain reliable data
about either the products they are evaluating (or components thereof), but most
often they do not in and of themselves provide a means to evaluate a finished
product in its actual end-use environment. It is also important to understand the
difference between a performance standard and a testing standard—the former
actually specifies values (strength, ductility, environmental resistance) that a
product must achieve, whereas the latter simply describes how a test to measure
a parameter should be conducted. It is the engineer’s job to use the correct
testing procedures from those that have been approved and on which he or she
can rely. Or, alternatively, if no approved test exists, the engineer must create
one that is reproducible, repeatable, reliable, and efficacious. Furthermore, it
is the engineer’s job to ensure the relevance of such testing to the overall and
final product performance in its end-use environment. No testing or standards
organization can foresee, nor do they claim to do so, all possible combinations
of product components, design choices, and functional end-use requirements.
Therefore, testing of a design in accordance with a testing standard does not
necessarily validate the design, nor does it necessarily mean that the design will
function in its end-use environment.
After testing and validation are complete, and the product is introduced to
the market, the design process is still not finished. As field experience is gained,
and products are used by consumers and sometimes returned to the manufacturer,
engineers often fine-tune and perfect designs based on newly acquired data. In
this part of the design process, engineers will analyze failures and performance
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problems in products returned from the field, and adjust product parameters
appropriately.8 The process of continual product improvement, illustrated by an
arrow from the “Go” stage to the “Design/Formulate” and “Test/Validate” stages
in Figure 1, is taught to engineers as a method to effectively optimize designs.
Such refinements of product design are often the topic of inquiry in depositions
of engineers and others involved in product design, and frequently misunderstood
as an indication that the initial design was defective.9 The engineering design pro-
cess anticipates review and ongoing refinement of product design as a means of
developing better and safer products. In fact, retrospective product modification
is mandated as company practice in some industries, and regulated or suggested
by the government in others. For example, examination of FDA guidelines for
medical device design will show a process that mirrors the one described above.
Another important component of the design process relates to changes in
technology that render a design, design feature, or even tools used by an engineer
obsolete. Engineers consider obsolescence to be a consequence of advancement,
and readily adjust designs, or create new designs, as new technology becomes
available. This concept is apparent in the automotive industry, where tremendous
advances in restraint systems and impact protection have greatly reduced the
risk of fatal injuries from driving (see discussion below). Although vehicles with
lap belts as the sole means of occupant protection would today be considered
unacceptable, they were by no means deficient when introduced in the 1950s.
From the engineer’s perspective, errors and omissions in the design process can
render a design defective; however, changes in technology can render a design
obsolete, not retrospectively defective.
Of course even well-designed products can fail, especially if they are not man-
ufactured or used in the manner intended by the design engineer. For example,
a steel structure may be adequately designed, but if the welds holding it together
are not properly made, the structure can fail. Similarly, a well-designed plastic
component manufactured in such a way as to overheat and degrade its constituents
may also be prone to premature failure. In terms of misuse of a product, most
engineers are trained to consider foreseeable misuse as part of the design process,
and one can generally expect to encounter a debate over what is reasonably fore-
seeable and what is not.
8. Although feedback on product performance and failure analysis on returned products is most
often used to perfect designs, the iterative nature of the process can also cause the design to progress
toward failure when cost becomes the driving factor.
9. Although the reasons for subsequent refinements in product design may be explored in
depositions, Federal Rule of Evidence 407 bars the introduction of evidence of such improvements at
trial as evidence of a defect in a product or a product’s design.
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The phrase “standard of care” has various meanings and connotations to
engineers that are somewhat discipline specific. Standard of care in the medical
sciences may be different than standard of care in some other context. In engineer-
ing, it can be said that the standard of care is met whenever the design process
was properly employed at the point in time that the event or incident happened.
Although the design process itself is “fixed,” when properly applied to a problem
in the 1940s and again to the same problem in 2009, the design outcome can be
quite different and indeed might be expected to be so. Even so, the standard of
care may be met each time.
3. State of the art
“State of the art” has a specific meaning in the law and may be the subject of a
particular statute in many jurisdictions. In addition, state of the art can be a dis-
tinct defense in many states.91 To engineers, however, its meaning may be slightly
different.
Simply put, this phrase refers to the current stage of development of a par-
ticular technology or technological application. It does not imply that it is the best
one can ever hope for but is merely a statement that at whatever point in time
referenced, technology was in a certain condition or form. For instance, the Intel
4004 4-bit microprocessor was state of the art in 1971 whereas the Intel 64-bit
microprocessor was the state of the art in 2006. Of course, there is the question
as to whether in either of these cases those microprocessors were state of the art
for just Intel, for all American semiconductor companies, or for all semiconductor
companies in the world. The question of the context in which this phrase is used
often lies at the heart of disputes. Because appropriate context may be difficult
to pin down, experts are often challenged with defining the “state of the art” in
relation to a particular technology or application. The answer from an engineering
perspective is often an assumption, nothing more, nothing less. As such, from an
engineering perspective, it is best to accept this phrase as a general colloquialism
that is difficult to define even though it is simple to state.
4. Best practice
Although this term is used colloquially and oftentimes in “business” activities, to
engineers it is not a phrase that is easily quantifiable and suffers from meaning
different things to different people. Despite this, it generally refers to the notion
that at any point in time there exists a method, technique, or process that is pre-
ferred over any other to deliver a particular outcome. That being said, there is
great latitude in how one goes about determining that preference and associating
it with the desired outcome. So, although it sounds good, this phrase is fraught
91. See, e.g., Ariz. Rev. Stat. § 12-683(1) (2009); Colo. Rev. Stat. § 13-21-403(1)(a) (2009);
Ind. Code § 34-20-5-1(1) (2009).
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with ambiguity. In the end, the more important issue is whether there was adher-
ence to the design process.
5. Regulations, standards, and codes
An issue that often arises in matters involving buildings and structures is the
distinction between design codes and physics (political laws vs. physical laws) in
the context of failure analysis. Design codes and standards are very conservative
political documents. They are conservative because they are intended to address
the worst-case scenario with a comfortable margin of safety. But buildings do not
fail because of code violations—they fail according to the laws of physics. They
do not fail when the code-prescribed loads exceed the code-prescribed strength
of the materials—they fail when the actual imposed loads exceed the physical
strength of the components. Buildings fail not when the laws of man are ignored
but when the laws of physics are violated. Examples of this are most common in
the context of earthquake-damaged structures. Buildings are not designed to resist
100% of expected earthquake forces. Rather, they are designed to resist only a
fraction of the expected load (typically about one-eighth) without permanently
deforming. The code implicitly recognizes that buildings are much stronger than
assumed in design and also have considerable ability to absorb overloads without
failure or collapse. Yet following an earthquake, engineers may inappropriately
compare the ground accelerations recorded by the U.S. Geological Survey with
design values in the code.
In the Northridge, California earthquake, recorded acceleration values were
2–3 times greater than the design code values. Many engineers concluded that the
buildings had been “overstressed” by 200–300% and were thus extensively dam-
aged, even if that damage was not visually apparent. In a line of reasoning remark-
ably similar to that of the plaintiff’s expert in Kumho,92 the damage was “proved”
analytically, even though it could not be physically seen (or disproved) in the
building itself. (If the same logic were applied to cars, every car that sustained an
impact greater than the design capacity of the bumper would be a total loss.) If this
approach was accepted, the determination of damage could only be done by a few
wizards with supposedly sophisticated, yet often unproven, analytical tools. The
technical issues in the Northridge situation were thus removed from the realm of
observation and common sense (where a jury has a chance of understanding the
issues) to the realm of arcane analysis where the experts have the final say.
This is not to say that standards and codes do not have their place in the
courtroom. We described above how standards are often used by engineers to
conduct tests, and cases that involve malpractice or standard-of-care may often
critically examine if a particular code was followed in the course of a design. On
92. 526 U.S. 137 (1999). In Kuhmo, the expert inferred the defect from an alleged set of
conditions, even though the alleged defect was not observed.
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the other hand, failure to use a code, or comparison of code values to actual values
does not guarantee that a disaster will occur. Common sense is often the best judge
in these situations—if a code value is exceeded, yet no damage is observed, it is
likely that the conservative nature of the code met its objectives and protected
its subject.
6. Other similar incidents
From an evidentiary perspective, evidence of similar or like circumstances has
a number of evidentiary hurdles to overcome before it can be admitted into
evidence.93 To an engineer, however, the concept of similarity or “other similar
incidents” (OSIs) has a somewhat different meeting and describes the types of
circumstances and documentation of such circumstances that an engineer can
rely on as a basis for his or her opinions. Although this section focuses primarily
on product design issues, the underlying theme is nonetheless broadly applicable
across the domain of engineering forensics.
Sometimes these other events are recorded in documentary form and relate
to events regarding product performance characteristics, product failures, prod-
uct anomalies, product performance anomalies, operational problems associated
with product use, product malfunctions, or other types of product failures. These
events are sometimes alleged by a party to a dispute to be substantially similar in
kind to an event or circumstance that had precipitated the subject case. Alleged
OSIs can be documented in multiple forms: (1) written narratives from various
sources (consumers, employees of the manufacturer, bystanders to a reported
event, insurers’ representatives, investigators, law enforcement personnel, owners
of a location involved in the dispute at bar, etc.) who might prepare and submit
a record of observation to a legal entity who retains those records of submission;
(2) telephonic reports of the same character and source as written reports, but
documented through telephone reports made to a recording representative or
office staff responsible for collecting event reports of interest to a legal entity;
(3) electronic submissions of the same character and source as written narratives;
(4) reports in a standardized format that are intended to record and document
events of interest (the forms may be in written or electronic media; (5) images of
events in film or electronic media that may or may not also have been recorded
and submitted in alternative formats. As a result, each may have its evidentiary
hurdles to overcome before it is admitted into evidence.
Similarly, each OSI may have legal issues regarding authentication, which
may be overcome by the repository where the underlying documentation is
93. For evidence of other similar incidents (OSIs) to be admissible, the proponent must show
that the OSIs are (1) relevant, see Fed. R. Evid. 401; (2) “substantially similar” to the defect alleged in
the case at bar; and (3) the probative value of the evidence outweighs its prejudicial effect, see Fed. R.
Evid. 403. Some courts merge the first two requirements; to be relevant, the OSIs must be substantially
similar to the incident at issue.
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found. The repositories of documents and reports that may be alleged to be OSIs
to an issue at bar can have many original purposes, and a collection of such docu-
ments may serve multiple purposes for the owner institution. Such document
collections may be used by the owner of the repository for various administrative
purposes, accounting, claims management and resolution, an archive of informa-
tion and/or data, database management, institutional knowledge building, war-
ranty management, in-service technology performance assessment and discovery,
service records, customer interactions, and satisfaction of regulatory specifications
or requirements, to name a few. Discovery requests may call for the owner of
the materials to search and retrieve records, documents, and reports from such
repositories even if the collections and repositories themselves may not have
been constructed for the purposes of document search and retrieval. Sometimes
engineers can be of use in searching and retrieving potentially relevant materials.
OSIs are discovered and may be offered into evidence to (1) demonstrate
prior knowledge on the part of the record owner regarding an alleged defect or
danger manifest to the consuming public that is causally related to the issue at bar;
(2) demonstrate by the number, volume, or rate of reports that a defect exists; and/
or (3) demonstrate careless disregard for the safety of others.94 To be admitted or
relied upon by an engineering expert, the proponent must demonstrate that the
event recorded and reported is “substantially similar” to the issue at bar.95 Testify-
ing engineers can be useful in identifying and describing the specific characteristics
that must be known and shown to make an assessment of similarity, including
specifying objective parameters for determinations of the degree of similarity or
dissimilarity and detailing the objective parameters and physical measurements
necessary and sufficient to determine substantial similarity. The conditions that are
necessary and sufficient to demonstrate substantial similarity include the following:
(1) the product or circumstance in the alleged OSI must be of like design to the
product or condition at issue in the instant case; (2) the product or circumstance in
the alleged OSI must be of like function to the product or condition at issue in the
instant case; (3) the application to which the product had been subjected must be
like the application to which the product at issue in the instant case was subjected;
and (4) the condition of the product, its state of repair, and/or its relevant state of
wear must be like the state of repair and the relevant state of wear of the product
that had been involved in the instant case.96 Engineers can contribute to a techni-
cal understanding of each of these dimensions and, in some cases, they may be able
94. See, e.g., Sparks v. Mena, No. E2006-02473-COA-R3-CV, 2008 WL 341441, at *2 (Tenn.
Ct. App. Feb. 6, 2008); Francis H. Hare, Jr. & Mitchell K. Shelly, The Admissibility of Other Similar
Incident Evidence: A Three-Step Approach, 15 Am. J. Trial Advoc. 541, 544–45 (1992).
95. See, e.g., Bitler v. A.O. Smith Corp., 391 F.3d 1114, 1126 (10th Cir. 2004); Whaley v. CSX
Transp. Inc., 609 S.E.2d 286, 300 (S.C. 2005); Cottrell, Inc. v. Williams, 596 S.E.2d 789, 793–94
(Ga. Ct. App. 2004).
96. See, e.g., Brazos River Auth. v. GE Ionics, Inc., 469 F.3d 416, 427 (5th Cir. 2006); Steele
v. Evenflo Co., 147 S.W.3d 781, 793 (Mo. Ct. App. 2004).
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to apply objective measures to questions of substantial similarity and thus quantify
the level of similarity between an event proffered as an OSI and the instant case.
The reverse is also true. Failure to establish likeness in any of these dimensions
is failure to demonstrate substantial similarity to the circumstances of the subject
case.97 If one or more of the necessary and sufficient conditions are unknown or
unknowable, the test of substantial similarity also fails; the lack of demonstrable
similarity is a lack of substantial similarity.
To demonstrate like design, a product or condition need not be identical in
all aspects of form.98 It must simply be similar in form to the product or condition
at issue in the instant case.99 Consider a machine control design with a feature
alleged to have been the proximate cause of an injury-producing event that gave
rise to a product liability lawsuit. Events proffered as OSIs that involve products
having an identical control design meet the test of “likeness” in design. In addi-
tion, other control designs that differ in aspects not related to the feature that is
alleged to have served as the proximate cause for the instant injury event may also
be considered to be “like” if the relevant design elements on the two products
cannot be differentiated. Engineers can assess the design elements of the control,
determine which features may be relevant to questions of design likeness, and
provide testimony to answer such questions.
Like function can be demonstrated if the operational purpose of the product
or condition defined in the alleged OSI is similar to the function of the product or
condition in the instant case. In the control design hypothesized above, a control
that is applied to command the dichotomous functional states to start and stop
(either “on” or “off”) a crane winch might serve the same operational purpose to
start and stop another type of equipment or winch. In such a case, the functions
and purpose of the control design may be alike. If however, that same control
design is applied to a machine in which the operational purpose is not simply to
command a dichotomous “on” or “off” signal, but rather its purpose is to provide
a modulated signal to which the machine response is a continuously variable func-
tion of control placement, the control design function is unlike the purpose of
dichotomous positioning. Engineers can provide assessments and analyses of the
functions embedded in a specific design and assist in the determination of likeness
or lack of likeness between an instant condition and one proffered as an OSI.
Like application can be demonstrated if it can be shown that the operational
conditions to which the product is subject are alike in the proffered OSI and in
the instant case.100 The environmental exposure to which a product is subjected
must be of like condition. A control design function can vary with temperature,
97. See, e.g., Peters v. Nissan Forklift Corp. N. Am., No. 06-2880, 2008 WL 262552, at *2
(E.D. La. Feb. 1, 2008); Whaley v. CSX Transp., Inc., 609 S.E.2d 286, 300 (S.C. 2005).
98. See, e.g., Bitler v. A.O. Smith, 391 F.3d 1114, 1126 (10th Cir. 2004).
99. Id.
100. See, e.g., Steele v. Evenflo Co., 147 S.W.3d 781, 793 (Mo. Ct. App. 2004).
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air or water exposure, reactions to corrosive elements, reactions to acid or base
contaminants, and in potential interactions with surrounding materials and com-
ponents that can be of differing electrochemical potential. Engineers with the
appropriate technical background can evaluate operating conditional applications
and determine if the conditions that obtain for a proffered OSI are similar to
those that had obtained in an instant case, thereby assisting the determination of
substantial similarity.
Differing environmental exposures resulting from differing applications may
render an event proffered as an OSI unlike and not substantially similar. Further,
like applications must comprehend that the load and stress conditions to which
a product or condition is placed is substantially similar to the circumstances that
obtained in the instant case to which the OSIs are being proffered for comparison.
In our control design identified above, the control device may be manually actu-
ated through a lever. Levers of differing length will apply differing forces to the
control device and produce differing operational stresses upon the control device
itself. The durability and performance of the control design itself can be affected
by these differing operating applications, and anomalies or failures under one
application may not be at all similar to those that obtain under differing circum-
stances in which the operating loads and applied stresses are different. Engineers
are well qualified to assess conditions of comparative loading and applied stresses.
A like state of repair can be demonstrated if there is reasonable evidence
that products involved in the proffered OSI are (1) in a specific working order,
(2) in a condition of adjustment (if possible to adjust), (3) in a state of wear, and
(4) within an expected range of tolerance that would not differentiate the product
or condition from that which obtained in the product or condition involved in
the instant case. Additionally, the products or conditions reported in the prof-
fered OSIs must be shown to be free of modification from an original design
state, or must be shown to be in a state of modification that is reflective of the
product or condition involved in the instant case.101 An absence of evidence
to demonstrate a state of likeness in application, operating environment, state
of repair and wear, or state of modification is not sufficient to show similarity.
Engineers with appropriate background can review data and information about
modifications and service conditions related to wear and wear rates, as well as
assess information related to the state of repair or disrepair, and thereby contribute
to understanding of the level of similarity or dissimilarity among specific events
and operational conditions.
For evidentiary reasons, OSIs generally are not admissible to demonstrate
the truth of the matter recorded therein.102 Event records are necessarily reports
of noteworthy events made after the fact by parties who may or may not have
an interest in establishing a specific fact pattern, may or may not be qualified to
101. See, e.g., Cottrell, Inc. v. Williams, 596 S.E.2d 789, 791, 794 (Ga. Ct. App. 2004).
102. Fed. R. Evid. 801 & 802.
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make the observations and assertions included in such reports, and may or may
not have any specialized training necessary to evaluate proper system function or
state of repair. The persons who report events collected and offered as OSIs may
not be fully informed of the set of circumstantial conditions that are necessary
and sufficient to determine causation of the reported event. Thus, often-reported
events have incomplete or insufficient data and information to determine sub-
stantial similarity. Even if informed, persons reporting events may not have the
correct observational powers, tools, and insights necessary for accurate evaluation
and reporting. The individuals who make reports regarding recorded events may
be unable to factually assess and accurately report all of the conditions relevant to
determination of event causation and resolution of questions regarding substantial
similarity. Reports of events made by parties who may have an interest in eco-
nomic recovery or other compensation may not always accurately disclose known
or knowable facts that could bear on determinations of causation and substantial
similarity. Furthermore, some parties may have an economic or other interest in
the outcome of a report or claim. Therefore, such reports, if offered to prove the
truth of the other incidents, are typically excluded as hearsay (unless the business
records exception applies).103
B. Demonstratives and Simulations
Computer animations, simulation models, and digital displays have become
more common in television and movies, especially in entertainment media con-
cerning forensic investigation, law enforcement, and legal drama. The result is
an increased expectation among the court and juries that visual graphics and dis-
plays will be used by engineering experts and other expert witnesses to explain
and illustrate their testimony. Additionally, boxed presentation software such
as PowerPoint, is often a technology used. Attorneys and their clients typically
expect their experts will use computer animations, simulations, and/or exhibits
to educate the jury and demonstrate the bases for their opinions. When used
correctly, these tools can make the expert’s testimony understandable and can
leave a lasting impression with the trier of fact of that party’s theory of the case.
For that very reason, the role of the court as the gatekeeper for use of these
demonstratives has become increasingly critical. As the technology underlying
these tools rapidly advances, the court’s task likewise becomes more difficult. In
assessing the validity of these tools, the court is often forced to decide whether
the visual display accurately represents the evidence and/or is supported by the
103. See Willis v. Kia Motors Corp., No. 2:07CV062-PA, 2009 WL 2351766 (N.D. Miss.
July 29, 2009) (finding customer complaints of similar accidents were not hearsay because they were
offered to notice, not the truth of the matter asserted, and even if they were hearsay, they fell under
the business records exception of Fed. R. Evid. 803(6)).
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expert’s opinions and qualifications.104 To assist the court in this difficult task,
we present some guidance regarding the types of technology presently in use
and the strengths and weaknesses of each.
A primary basis for misunderstanding and uncertainty is the difference
between a computer animation and a computer simulation. An animation is a
sequence of still images graphically illustrated (two dimensions) or modeled (three
dimensions), and are often textured and rendered to create the illusion of motion.
A cartoon is a simple example. There are no constraints inherent in an animation,
and the laws of physics, or any other science, do not necessarily apply (a black
mouse can be dressed in red shorts with yellow shoes and be made to dance, sing,
and fly). The lack of imposed restriction does not make the animation deficient a
priori; if the still images that comprise the animation are accurate in their repre-
sentation of individual snapshots of time, then the animation itself can be proven
precise. The converse, of course, is also true.
Animations contain key frames that define the starting and ending points
of actions, with sequences of intermediate frames defining how movements are
depicted. For example, a series of still photographs can depict the path of a vehicle
vaulting off an embankment, with a single image at the takeoff, mid-flight, and
landing positions each correct in its representation. However, when an animation
of the event is created, the intermediate frames fill in the missing areas, and if so
desired, contrary to known physical phenomena, the animation could show the
vault trajectory of the vehicle to remain flat and then suddenly drop, similar to
the inaccurate representation of motion experienced by a cartoon coyote momen-
tarily contemplating his fate after chasing a bird off a cliff. Thus, in an animation,
some of the inputs (stills) may represent reality, but the sum of the parts (inter-
mediate frames) may not.
Unlike an animation, a simulation is a computer program that relies on
source data and algorithms to mathematically model a particular system (see, e.g.,
the discussion on finite element modeling, above), and allows the user to rapidly
and inexpensively gain insight into the operation and sensitivity of that system
to certain constraints. Perhaps the most common example of a simulation can be
found daily as a computer-generated image showing the predicted growth of a
storm system.
On the surface, a simulation would seem to provide more accuracy than an
animation. However, this is not necessarily the case. The simulation model is only
as accurate as its input data and/or constraining variables and the equations that
form its calculation stream. Simulation models also require a sensitivity analysis—
just because a model produces an answer does not mean that it is the best model or
104. See Lorraine v. Market Am. Ins. Co., 241 F.R.D. 534 (D. Md. 2007) (distinguishing
between demonstrative computer animations and scientific computer simulations and discussing the
evidentiary requirements, including authentication, for each); People v. Cauley, 32 P.3d 602 (Colo.
Ct. App. 2001) (same).
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the most correct answer. For example, a computer model depicting the motions of
a vehicle prior to and after an impact with a pole may be correct if it matches the
known physical evidence (e.g., tire marks and vehicle damage). However, whether
the model is accurate depends on the accuracy of the inputs for tire friction, vehicle
stiffness, vehicle weight, location of the vehicle’s center of gravity, etc. Even if the
inputs are accurate, once a solution is found, other solutions may exist that also
match the evidence. Assessing the accuracy of each solution requires an iterative
process of making changes to those variables believed to have the greatest effect
on the output. Simply put, the difference between a vehicle accident simulation
model that predicts 10 inches of crush deformation and two complete revolutions
post impact versus 14 inches of crush and three complete revolutions may depend
on just a few selected vehicle characteristics. Thus, compared to an animation, in
a simulation model, the sum of the constraining variables and equations may rep-
resent reality, but some of the user-selected inputs may not.
The difficulty for the court is the need to decide whether some or all of the
computer animation or simulation accurately represents the facts and/or opinions
of the expert.105 This is not an easy endeavor, but can usually be executed in a
reasonable fashion for simulations by evaluating whether the simulation has been
validated. If the underlying program predicts the behavior of vehicles in a crash, it
can be validated by crashing vehicles under controlled conditions, and comparing
the actual results to those predicted by the simulation. If the software in question
predicts the response of a complex object to applied forces, it can be validated by
modeling a simple object, the response of which can be calculated by hand, and
comparing the simulation to those known results.106
Similarly, for animations, engineers need to establish authenticity, relevance,
and accuracy in representing the evidence using visual means.107 They may rely
on blueprint drawings, CAD (computer-aided design) drawings, U.S. Geological
Survey data, photogrammetry, geometric databases (vehicles, aircraft, etc.), eye-
witness statements, and field measurements to establish accuracy of an animation.
VIII. Epilogue
Most engineers are not educated in the law and to them the setting of a deposi-
tion or a courtroom is peculiar and often uncomfortable. The rules are different
105. See id.
106. See Lorraine v. Markel Am. Ins. Co., 241 F.R.D. 534 (D. Md. 2007); Livingston v. Isuzu
Motors, Ltd., 910 F. Supp. 1473 (D. Mont. 1995) (finding computer simulation of rollover accident
by expert to be reliable and admissible under Daubert whether computer program was made up of
various physical laws and equations commonly understood in science, program included case-specific
data, and expert’s computer simulation methodology had been peer reviewed).
107. See, e.g., Friend v. Time Mfg. Co., No. 03-343-TUC-CKL, 2006, WL 2135807 (D. Ariz.
July 28, 2006); People v. Cauley, 32 P.3d 602 (Colo. Ct. App. 2001).
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from those to which they are accustomed. The conversations are somewhat alien.
Treading in this unfamiliar territory is a challenge. And so, although it is impor-
tant for the engineer to “fit” into this environment, it is equally important for
the triers of fact and the court to understand the engineer’s world. We hope this
chapter has provided a glimpse into that world, and by considering it, the reader
will have some insight as to why engineers respond to questions as they do. The
foundation that underlies and supports essentially all that has been done and all that
will be done by engineers is the design process. It is the roadmap for innovation,
invention, and reduction to practice that characterizes those who do engineering
and who call themselves “engineers.” It is the key metric against which products
and processes can be and should be evaluated.
IX. Acknowledgments
The authors would like to thank the following for their significant contributions:
Dr. Roger McCarthy, Robert Lange, Dr. Catherine Corrigan, Dr. John Osteraas,
Michael Kuzel, Dr. Shukri Souri, Dr. Stephen Werner, Dr. Robert Caligiuri,
Jeffrey Croteau, Kerri Atencio, and Jess Dance.
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