The statement of task to the committee includes the following: “Evaluate the scope of characterization testing relative to the benefit of the information obtained.” The term “characterization” is broad and is used in different ways in different contexts. However, the Office of the Director, Operational Test and Evaluation (DOT&E) provided additional information to elaborate on this task. Most of the issues raised by DOT&E that are relevant to this portion of the statement of task are addressed in this chapter. The committee also describes additional characterization tests that are needed. Some of these are intended for future helmet designs. A number of these additional tests have been discussed in earlier chapters and are repeated here because they can be viewed as being related to characterization studies. These include the following: evaluating helmet performance across a broader range of, and more realistic, threats; assessing the effect of aging; understanding the relationship between helmet offsets and helmet protection; and conducting gauge repeatability and reproducibility (R&R) studies to understand the different sources of variation in the test process and possibly providing opportunities to reduce some of the variation. This chapter also includes a discussion of current V50 testing and an alternative methodology as well as a discussion of industrial practices in characterizing process capability.
The committee’s task to “evaluate the scope of characterization testing relative to the benefit of the information obtained” was added after the committee had started its deliberations, apparently in response to issues raised in the Department of Defense (DoD) Inspector General Report (DoD IG, 2013).
Chris Moosmann from DOT&E provided additional information on the task during a presentation to the committee on March 21-22, 2013. He said:
• ACH (Advanced Combat Helmet) characterization was not done prior to release of the helmet test protocol;
• DOT&E and PEO (Program Executive Office) Soldier have committed to characterize ACH helmets;
• DOT&E indicated that the ECH (Enhanced Combat Helmet) would also be characterized;
• DOT&E will use the results of characterization to determine whether any changes to current protocol standards are appropriate; and
• DOT&E/program offices will consider characterization of new future designs during developmental testing to assess any need for protocol changes.1
Mr. Moosmann’s presentation noted that the following questions will be addressed as part of the above characterization testing:
1. What is the lower confidence limit (90% confidence) on P(nP) as measured with n shots?
2. What percent of the population (90% confidence) meets the backface deformation (BFD) requirement by location?
3. Do shot location, helmet size, environment, and shot sequence affect P(nP) or BFD?
4. What effect do shot location, helmet size, and shot sequence have on the slope of the ballistic characterization curve?
5. What are the V0 and V50 velocities associated with the fragment simulating projectiles (FSPs) and right circular cylinders (RCCs) currently used during helmet testing?
1Chris Moosmann, Live Fire Test & Evaluation, DOT&E, “DOT&E Issues Update,” presentation to the committee on March 21, 2013.
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9 Characterization Tests for the Advanced Combat Helmet and Future Helmets 9.0 SUMMARY Chris Moosmann from DOT&E provided additional information on the task during a presentation to the commit- The statement of task to the committee includes the tee on March 21-22, 2013. He said: following: “Evaluate the scope of characterization testing relative to the benefit of the information obtained.” The term • ACH (Advanced Combat Helmet) characterization “characterization” is broad and is used in different ways was not done prior to release of the helmet test in different contexts. However, the Office of the Director, protocol; Operational Test and Evaluation (DOT&E) provided addi- • DOT&E and PEO (Program Executive Office) Sol- tional information to elaborate on this task. Most of the dier have committed to characterize ACH helmets; issues raised by DOT&E that are relevant to this portion of • DOT&E indicated that the ECH (Enhanced Combat the statement of task are addressed in this chapter. The com- Helmet) would also be characterized; mittee also describes additional characterization tests that are • DOT&E will use the results of characterization to needed. Some of these are intended for future helmet designs. determine whether any changes to current protocol A number of these additional tests have been discussed in standards are appropriate; and earlier chapters and are repeated here because they can be • DOT&E/program offices will consider characteriza- viewed as being related to characterization studies. These tion of new future designs during developmental include the following: evaluating helmet performance across testing to assess any need for protocol changes.1 a broader range of, and more realistic, threats; assessing the effect of aging; understanding the relationship between Mr. Moosmann’s presentation noted that the following helmet offsets and helmet protection; and conducting gauge questions will be addressed as part of the above character- repeatability and reproducibility (R&R) studies to under- ization testing: stand the different sources of variation in the test process and possibly providing opportunities to reduce some of the 1. What is the lower confidence limit (90% confidence) on variation. This chapter also includes a discussion of current P(nP) as measured with n shots? V50 testing and an alternative methodology as well as a 2. What percent of the population (90% confidence) meets discussion of industrial practices in characterizing process the backface deformation (BFD) requirement by loca- capability. tion? 3. Do shot location, helmet size, environment, and shot sequence affect P(nP) or BFD? 9.1 INTRODUCTION 4. What effect do shot location, helmet size, and shot se- The committee’s task to “evaluate the scope of charac- quence have on the slope of the ballistic characterization curve? terization testing relative to the benefit of the information 5. What are the V0 and V50 velocities associated with the obtained” was added after the committee had started its fragment simulating projectiles (FSPs) and right circular deliberations, apparently in response to issues raised in the cylinders (RCCs) currently used during helmet testing? Department of Defense (DoD) Inspector General Report (DoD IG, 2013). 1ChrisMoosmann, Live Fire Test & Evaluation, DOT&E, “DOT&E Is- sues Update,” presentation to the committee on March 21, 2013. 64
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CHARACTERIZATION TESTS FOR THE ADVANCED COMBAT HELMETS AND FUTURE HELMETS 65 6. What BFDs are associated with FSPs/RCCs currently Similarly, existing data for ACH can be used to answer used during helmet testing? Questions 3 and 4 above. The suite of resistance-to-pene- 7. How do helmets perform against foreign threats?2 (slide 5) tration (RTP)/BFD tests for FAT (see Table 4-1) consists of a designed “full factorial experiment” with three factors: The presentation requested that “the committee review helmet size (Small, Medium, Large, Extra Large), condition- and comment on the scope of characterization testing relative ing environment (ambient, hot, and cold temperatures, and to the benefit of the information obtained and the resources seawater), and shot location (front, back, left, right, crown). required to do so.” In particular, While the procurement decision rules are based on aggre- gated data, the full data provide the necessary information I. Are there additional questions that should be addressed to characterize differences among helmet size, shot location, (threats, conditions, etc.)? and environment, as specified in Questions 3 and 4 above. In II. Should characterization address issues such as durability fact, Chapter 5 (Section 5.3) reports some answers to these and aging (“shelf life”)? III. Should there be a common (minimum) set of questions all questions from the committee’s analyses of FAT and LAT characterization efforts should address and what should data that were made available to it. Moreover, the “cluster- those include?3 (slide 6) ing” analysis already being done by DOT&E and the Institute for Defense Analysis is aimed at characterizing exactly these The rest of this chapter is aimed at identifying the relevant differences.4,5 aspects of characterization, addressing the questions posed The current goal of the clustering analysis is to do pre- by DOT&E, and providing a general discussion of industrial liminary tests to see if the data can be pooled across the practices involved in studying process capability. different factors (environment, locations, etc.), and the com- mittee has noted in Chapter 7 that such preliminary tests are not to be recommended. However, the analyses to estimate 9.2 CHARACTERIZATION OF THE ADVANCED the differences among the factors and to monitor them over COMBAT HELMET USING EXISTING TEST DATA time (Questions 3 and 4 above) are certainly important and For the ACH, existing test data from first article testing should be continued. (FAT), lot acceptance testing (LAT), and other sources can V50 testing, raised in Question 5, is discussed in Section be used to answer most of the questions posed above by 9.4 in this chapter. Regarding Question 6, the committee DOT&E. In fact, Question 1 was the subject of Recom- does not know if data from fragment simulating projectiles mendation 6-3 in Chapter 6. It notes that upper confidence (FSPs) and right circular cylinders (RCCs) are stored from bounds (UCBs) should be computed and reported based on past FAT studies for ACH. If they are, Question 6 can also the observed number of penetrations in FAT. In addition to be readily answered. characterizing the actual penetration probability, the UCBs The issue of testing helmets against other threats has can be used to monitor how the penetration levels vary over been discussed extensively in the report. The committee will time and among manufacturers. The same kinds of analyses return to this point in Section 9.3. should also be done with LAT data to monitor a manufac- ACH test data can also be used to characterize many other turer’s performance over time. aspects of helmet performance. For example, FAT and LAT A similar recommendation was made in relation to Ques- data can be compared over time to find trends and patterns tion 2 in Chapter 7. Recommendation 7-3 states that the BFD associated with the production process for an individual man- measurements (from FAT) should be analyzed to determine ufacturer. Data can also be compared across manufacturers the margins (number of standard deviations between the to detect possible differences across manufacturers. Further, mean BFD and its threshold) and tracked over time to assess data from the drop-tests can be used to track performance changes. Since the BFD thresholds lack scientific basis, it is of manufacturers over time in terms of blunt-force trauma. better to track changes in the margins or examine the exceed- ance probabilities at multiple thresholds. It is straightforward 9.3 EXPANDED CHARACTERIZATION REQUIRING to compute the point estimates and associated confidence ADDITIONAL DATA intervals (or upper bounds) for the exceedance probabili- ties. Again, similar analyses should be done with LAT data DOT&E also asked if there were additional topics that to track a manufacturer over time. Recommendation 7-4 should be part of its characterization studies. The committee suggests replacing the current ad hoc threshold for BFD (at describes selected topics here. This class of characterization different locations) using data-based limits obtained from historical BFD test data. Developing such limits can be 4Janice Hester, Research Staff Member, Institute for Defense Analysis, viewed as a characterization study. “DOT&E Helmet Test Protocols Overview: Statistical Considerations and Concerns,” presentation to the committee on January 25, 2013. 5Laura Freeman, Research Staff Member, Institute for Defense Analysis, 2Ibid. “Protocol Analyses and Statistical Issues Related to Testing Methodologies,” 3Ibid. presentation to the committee on March 21, 2013.
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66 REVIEW OF DEPARTMENT OF DEFENSE TEST PROTOCOLS FOR COMBAT HELMETS studies is intended to explore the properties of the helmet that many soldiers change the padding locations or beyond the current DOT&E protocol. Several of these sug- remove some of the pads from their helmets in the gestions are of a longer-term nature and intended for the field. Understanding the differences between test- ECH and newer generations of helmets rather than the ACH. ing results and what would be experienced by the soldier would help quantify relevance of the testing. • Evaluate helmet performance for a variety of dif- One option for such a characterization study would ferent threats. As noted in Chapter 3, the primary be to obtain samples of common pad configura- focus of DOT&E’s (and the Army’s) test protocols tions in the field and perform the standard RTP and is gunfire threats. Recommendations 3-1, 3-2, and BFD testing. This would allow better connection 3-5 emphasize the importance of expanding the test of results to soldier experience and may suggest profile to cover emerging threats as well as more real- additional recommendations or requirements for istic blunt-impact threats. For example, improvised soldiers. explosive devices (IEDs) have dramatically different • Evaluate the relationship between helmet offsets and distributions of fragment sizes and velocities com- helmet protection. With the availability of 5 headform pared to those from artillery. Recommendation 3-3 sizes, it should be straightforward to characterize asks DoD to reassess helmet requirements for cur- differences in BFD by location as a function of rent and potential future fragment threats, especially helmet offset. It is widely assumed that increased those energized by blast. Such a reassessment would offset provides improved protection through reduced include examining redundancy in the current profile BFD magnitude. (However, Figure 5-3 in Chapter 5 of threats, such as the 2-grain versus 4-grain, and may shows that this may not be the case.) Quantifying lead to elimination of some tests. Resources can then this improvement, if it exists, could lead to changes be redistributed to cover a wider range of realistic to helmet assignment or a reassessment of the trade- ballistic threats, including larger mass artillery frag- offs between functionality and protection. ments, bullets other than 9-mm, and IED fragments. • Evaluate the aging characteristics of the helmets to A comprehensive examination of threat profiles determine if there is any meaningful degradation of would involve considerable additional resources and the protection performance of the helmets over time. consist of much more than characterization studies. An approach to this testing might be to store some of Nevertheless, the committee believes that this is a the helmets from a given lot and perform a test simi- very important direction for future efforts by DoD. lar to FAT testing on helmets of different ages. For • Evaluate the sources of variation in the test process. example, if helmets were generally thought to be used As noted in Chapter 4, there are many sources of for 2 years before they were replaced, then a testing variation in the test process and test measurements. regimen could be established that tests helmets at Recommendation 4-2 recommends that the DoD ages 0, 6, 12, 18, 24, and 30 months to determine conduct formal gauge R&R studies to understand if there are changes in protection performance. An the different sources of variation (test methodology, alternative would be to develop an accelerated testing helmets, use of clay, headforms, etc.) and use the program in which the helmets are exposed to stressful results to improve the test process. The committee environmental or to use conditions that would simu- judges that this should be a high priority, given the late accelerated aging. This testing would provide high costs of testing and the benefits to be gained reassurances that the helmets are not degrading over from such an R&R study. time. • Evaluate helmet performance at selected areas of the helmet not currently tested. The test protocols do not Program and oversight personnel can identify other poten- assess the helmet in some regions, such as edges and tially important characterization tests that would provide around the ear covering. While it may be reasonable additional information about a helmet’s protective capabili- to exclude them in the formal test process, it is still ties. DOT&E’s charge to the committee specifically asked of importance to understand the range of protection for an evaluation of “the scope of characterization testing afforded at these helmet locations. Potential dif- relative to the information obtained.” The committee does ferences in manufacturing choices could be better not have the necessary information or the expertise to do understood and might lead to improvements in over- a cost-benefit analysis. On the other hand, the Department all design. of Defense has the relevant expertise and information as • Evaluate performance for different helmet pad con- to which information is important for soldier safety in the figurations. Current testing procedures test the five battlefield. DoD is better equipped to make the decision on locations with padding directly in the line of fire of which tests should be done, how to fund them, and whether the shot (crown, front, and back) or in a gap between funds should be redistributed from current test resources for pads (left and right). Anecdotal evidence suggests important characterization tests.
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CHARACTERIZATION TESTS FOR THE ADVANCED COMBAT HELMETS AND FUTURE HELMETS 67 Chris Moosmann’s presentation to the committee6 listed The V50 testing procedure under MIL-STD 662F is as some possible studies that are being planned to charac- follows: terize the ACH (from different vendors) and compare its performance with the lightweight ACH. If the ACH will no • A first round is shot with a striking velocity that is longer be procured (only current manufacturers who have approximately 75 to l00 feet per second (ft/s) above passed FAT will produce them), then it is not wise to invest the minimum V50 required per specification. (Previ- considerable additional resources to characterize the ACH. ous V50 testing on comparable helmets could also New tests and characterization studies should focus on new provide a good starting velocity.) helmet designs. • If the first round results in a complete penetration, When DoD adopts new helmets with changes to the the velocity of the second round is decreased by 50 design (such as lighter weight and added mobility), it will to 100 ft/s from the velocity of the first round. If it be necessary to reevaluate the test protocols. For example, results in no or partial penetration, the velocity is it may not be possible for manufacturers to produce lighter increased by 50 to 100 ft/s. helmets at current levels of penetration. • In subsequent shots, the velocity is increased or decreased, as applicable, until one partial and one Recommendation 9-1. When combat helmets with new complete penetration is obtained. designs are introduced, the Department of Defense should • After obtaining at least one partial and one complete conduct appropriate characterization studies and cost-benefit penetration, the velocity is increased or decreased in analyses to evaluate the design changes before making deci- increments of 50 ft/s. Firing is continued until suf- sions. It is not advisable to automatically apply the same ficient partial and complete penetrations are obtained standard (such as the 90/90 rule or others) when these tests to estimate V50 by taking the average of the velocities could potentially be across different protective equipment corresponding to an equal number of the highest par- (body armor, helmets, etc.), different numbers of tests (e.g., tial and the lowest complete penetration, as specified 96 tests for the enhanced combat helmet, 240 tests for the in the contract (DoD, 1997, p. 10).7 Typically 8-14 advanced combat helmet), or over time. shots are used. The committee notes that the protocol allows multiple shots 9.4 V50 TESTING per helmet, but it does not explicitly specify a maximum number of shots or shots per helmet: “If a valid V50 cannot Description be obtained with a single finished shell, the V50 will continue V50 refers to the “the velocity at which complete penetra- on an additional finished shell(s)” (IOP PED 003, Paragraph tion and incomplete penetration are equally likely to occur” 22.214.171.124). (DoD, 1997, p. 3). That is, V50 is the median of the velocity- penetration distribution or curve. (This is analogous to dose- Finding 9-1. The current V50 testing protocol does not response studies that arise in pharmaceutical studies.) This clearly specify the maximum number of shots per helmet. theoretical quantity is currently estimated from a series of ballistic tests using the methodology of Military Standard During the committee’s discussions with representatives (MIL-STD) 662F (DoD, 1997). of PEO Soldier8 (Lozano, 2013) and DOT&E, the following V50 testing is an important component of the overall reasons were given for collecting V50-related data: DOT&E protocol. The estimated value of V 50 is used informally to track and compare helmet performance. The • It is a commonly understood metric that characterizes nature of the test suite and the subsequent data analysis are the performance of the helmet, both in the United quite different from the RTP and BFD protocols. For these States and in member countries of the North Atlantic reasons, the committee considers V50 testing to be a part of Treaty Organization. characterization. • It is easier to estimate than potentially more relevant Table 4-1 (in Chapter 4) shows the test matrix and require- velocity quantities such as V0 or V10. ments for V50 testing under DOT&E’s FAT protocol. It is per- 7This formed for 2-, 4-, 16-, 17-, and 64-grain threats as well as a estimation methodology is similar to the NATO Standardization small arms threat (if required). The Army’s lightweight ACH Agreement (STANAG) 2920, Ballistic Test Method for Personal Armour Materials and Combat Clothing, promulgated 31 July 2003. STANAG 2920 Purchase Description (which also specifies MIL-STD-622) requires an even number of at least six shots, half of which perforate and half further requires that helmets achieve a minimum V50 for each of which do not, and all of which are have velocities of within 40 meters per of the fragmentation threats (U.S. Army, 2012). second. Then the V50 is estimated as the mean velocity of the shots meeting these conditions (NSA, 2003). 8Frank J. Lozano, Product Manager, Soldier Protective Equipment, “Set- 6ChrisMoosmann, Live Fire Test & Evaluation, DOT&E, “DOT&E Is- ting the Specifications for Ballistic Helmets,” presentation to the committee sues Update,” presentation to the committee on March 21, 2013. on April 25, 2013.
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68 REVIEW OF DEPARTMENT OF DEFENSE TEST PROTOCOLS FOR COMBAT HELMETS • It can be useful for comparing helmet performance Understanding the ability of a current product to conform between manufacturers and over time. to production requirements is a common aspect of indus- • PEO Soldier uses V50 time series data as a leading trial practice and product improvement and is often called indicator of manufacturer process degradation. capability analysis (Bothe, 1997; Pyzdek and Keller, 2003). It encompasses characterization of process stability as well V50 values are used informally. More structured analyses as margin on performance relative to product requirements could be done to compare V50 estimates among manufactur- (Hoerl and Snee, 2012). It is applicable to understanding ers, over time, and among environments. Another potential product conformance internal to a company and for external characterization analysis would be to investigate the relation- suppliers, customers, and users. Typically, formal product ship between V50 and fragment grain size. requirements such as acceptable failure rates and specifica- tion limits are based on understanding customer needs. In the helmet procurement process, this would likely be based Additional V50 Testing and Characterization Analyses on data collected during developmental testing. Developing The current goal of V50 testing is to estimate a single point a stronger connection to what is possible, given current hel- (the median) on the velocity-penetration curve. In the com- met manufacturing capability, would allow the opportunity mittee’s view, it would be beneficial to expand V50 testing to leverage this into improved helmets for the soldier. Using so that the whole curve can be estimated with reasonable legacy measures to define the standard a helmet is required precision, without expending a lot more additional resources to meet for FAT and LAT represents a lost opportunity and in terms of number of shots. potentially an important sacrifice in helmet protection. This expanded testing would involve taking multiple shots at different (selected) velocities and fitting a parametric curve Recommendation 9-3. To be consistent with the goal of to the velocity-penetration response data. Typical choices continuous improvement, developmental testing results from for the curve are logistic or normal distributions, leading to helmet design should be used to allow better calibration of logit and probit curves, respectively. This approach allows current helmet capability and to help define more meaningful for estimation of any quantile of the velocity-penetration thresholds for helmet protection. distribution, not just the median. One can also compute the standard error associated with the estimated quantile. There A key difference in DoD’s approach used in the procure- is extensive literature on the design and analysis of such ment process for helmets from the more common practice of studies (Ruburg, 1995; Prentice, 1976). industry is the focus on performance specifications instead The curves are typically described by two parameters for of design specifications. In much of industry, and indeed for location and shape. The shape parameter provides an indi- some military procurement processes involving complex cation of the spread in the velocity-penetration distribution. products and systems, when a product is being developed, It measures how consistent the penetration velocity is from design specifications for material, structure, and assembly helmet to helmet or among shot locations within a helmet. are the basis for assessing its adequacy. In other words, the Changes in a production process, for example, could either manufacturing process is closely monitored and checked to increase or decrease the variability of penetration velocities. make sure that the product matches the details for what is Certain environments might not affect V50 but could increase required. This provides a direct and easily measurable means the standard deviation and, thereby, degrade a helmet’s pro- of checking new products as they are completed. tective capability. On the other hand, the current DoD helmet procurement process allows manufacturers to build the helmet with any Recommendation 9-2. The Department of Defense should design specifications, and the sole test of the adequacy of consider alternative approaches to its current methodology the helmet is through performance tests during FAT and for estimating V50. One alternative is to estimate the entire LAT testing. An advantage of this approach is that it allows velocity-penetration distribution by varying the shot veloci- the manufacturers the flexibility to change the process and ties over a prescribed range. Given the limited test resources update their production methods as technology evolves. (number of shots), the estimation methodology has to be However, it has the disadvantage of placing all of the burden based on fitting parametric curves. The approach also allows for evaluation at the end of the production process through computation of standard errors associated with V50 and other rigorous and expensive testing. quantiles of interest. A potentially beneficial alternative—one that would encourage improved process monitoring while still allow- ing manufacturers flexibility to improve their product as 9.5 COMPARISON WITH INDUSTRIAL PRACTICES new technologies are developed—would be to combine the So far, this chapter has focused on specific issues on char- design and performance specification approaches. Manufac- acterization related to helmet testing. This section provides a turers could develop their own design specifications, which more general discussion of industrial best practices. would then be tracked with reports given to the DOT&E.
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CHARACTERIZATION TESTS FOR THE ADVANCED COMBAT HELMETS AND FUTURE HELMETS 69 This information would then be used to complement the bilities, relative to the costs and resources they require. A performance-based testing currently used, particularly at the number of the proposed characterization studies can be done LAT testing stage. This additional information would allow using data that are collected as part of the FAT and LAT test DOT&E to have better understanding of the stability of the process. Others will require different types of testing and the process, while having the reassurances of the performance- investment of additional resources. based testing. Once the design specification requirements have been Recommendation 9-5. For new generations of helmets, the determined by the manufacturer, then the capability of the scope of characterization studies should be broader than currently available product can be quantified using one of what is currently being done. They should include many of the common process capability metrics (Montgomery, 2012). the activities described in Section 9.3. In the absence of formally specified requirements, matching or surpassing current production capability is a common 9.7 REFERENCES alternative for capability analysis methods. Characterizing product performance is an established practice in industry Bothe, D.R. 1997. Measuring Process Capability: Techniques and Calcula- tions for Quality and Manufacturing Engineers. McGraw-Hill, New and is used to quantify current performance as well as York, N.Y. establish a baseline from which target future improvements DoD (Department of Defense). 1997. Department of Defense Test Method can be assessed. Standard: V50 Ballistic Test for Armor. MIL-STD-662F. U.S. Army The standard approach to monitoring stability of produc- Research Laboratory, Aberdeen Proving Ground, Md. tion is through control charts based on manufacturing char- DoD IG (Department of Defense Inspector General). 2013. Advanced Combat Helmet Technical Assessment. DODIG-2013-079. Department acteristics (Hoerl and Snee, 2012), that allow for continuous of Defense, Washington, D.C. supervision and monitoring of standards as products are Hoerl, R.W., and R.D. Snee. 2012. Statistical Thinking: Improving Business being produced. Supervision and monitoring involve active Performance. Wiley, Hoboken, N.J. management and watching real-time results to see if there Lozano, F., Product 9 Manager, Soldier Protective Equipment, U.S. Army. is a problem. Current FAT and LAT testing is based on a 2013. V50 Ballistic Limit Testing. Information paper. June 18, 2013. U.S. Army, Fort Belvoir, Va. paradigm of inspection, in which during post-production Montgomery, D.C. 2012. Introduction to Statistical Quality Control. Wiley, the products are evaluated to assess conformance. Standard Hoboken, N.J. practice in industry has evolved away from primarily using NSA (NATO Standardization Agency). 2003. Ballistic Test Method for Per- inspection to a model in which monitoring is a key aspect sonal Armour Materials and Combat Clothing. NSA/0723-PPS-2920. of ensuring ongoing product quality. Monitoring has the STANAG 2920 PPS–Edition 2. NATO Standardization Agency, Brus- sels, Belgium. advantages of ensuring that a production process operates at Prentice, R.L. 1976. Generalization of the probit and logit methods for dose its full potential, reducing waste, and detecting changes in response curves. Biometrika 32:761-768. performance quickly. Pyzdek, T., and P.A. Keller. 2003. Quality Engineering Handbook. CRC Press, Boca Raton, Fla. Recommendation 9-4. Manufacturers should be required to Ruburg, S.J. 1995. Dose response studies I. Some design considerations. Journal of Biopharmaceutical Statistics 5(1):1-14. provide some documentation of ongoing process monitoring U.S. Army. 2012. Advanced Combat Helmet (ACH) Purchase Description, of the helmet production as a beneficial enhancement to the Rev A with Change 4. AR/PD 10-02. Soldier Equipment, Program lot acceptance testing protocol. Executive Office—Soldier, Fort Belvoir, Va. 9.6 CONCLUDING REMARKS It is for DoD to choose the appropriate characterization tests and analyses that should be done, based on its assess- ment of the benefits, in terms of improving the understanding of helmet protective properties and improving those capa-