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Chapter 3
THE ASSESSMENT OF MOBILITY
The assessment of mobility is a central issue in any discussion of
approaches to more effective mobility aids. It is necessary for
determining the success or failure of mobility training and the
effectiveness of mobility aids and for developing improved techniques
and devices for mobility by the visually impaired. In this chapter we
review the advances that have been made in the assessment of mobility
performance and comment on the further refinements that will be
necessary to relate mobility performance measures to the design,
development, and use of mobility aids.
INDIVIDUAL CHARACTERISTICS
An individual's mobility performance is affected, of course, by a
multitude of predisposing characteristics. Examples of such factors
are listed in Table 4. To provide adequate mobility training to a
person in need of it, an instructor must consider psychosocial,
sensory/motor, perceptual/cognitive, and environmental factors in the
choice of instructional technique, the training procedure, and,
inevitably. the outcome of the trainina oroaram.
Past studies have examined the differences in assessment and
instructional techniques for specific subgroups of the visually
impaired population. For the two-thirds or more of the visually
impaired population who are 65 or older, travel needs may differ
significantly from those of a younger subgroup. Nonetheless, elderly
visually impaired persons have received relatively little attention
from the rehabilitation community (Allen et al., 1976, 1977; Carney,
1970; Hubbard, 1969; Shaw, 1970~. More attention has been directed to
the needs of children (Baird, 1977; Bledsoe, 1963; Ferrell, 1979;
Franks and Nolan, 1970, 1971; Goldie, 1977; Hapeman, 1967), the multiply
handicapped (Geruschat, 1980; Harley and Merbler, 1980; Primrose,
1981), and the partially sighted (Allen, 1977; Burton, 1976; Cory and
Prince, 1976; Hughes, 1967; Quillman, 1977~.
26
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TABLE 4 Examples of Predisposing Factors That Affect Mobility
Examples of Psychosocial Characteristics
Individual expectation
Living situation
Motivation
Demographic profile
Sensory/Motor Characteristics
Visual capacity
Auditory capacity
Locomotor capacity
Perceptual/Cognitive Characteristics
Ability to utilize echoes
Spatial awareness
Cognitive capacity/processing mode
Environmental Characteristics
Urban/suburban/rural
Weather conditions
Time of day
Previous training/learning
Familiarity
Psychosocial Characteristics
The psychosocial factors that affect mobility have not received the
attention they deserve. Those that have been studied include the
dynamics of the student-instructor interaction (Flaharty et al., 1976),
client motivation (Murphy, 1965), and self-perception (Schulz, 1972,
1977; Welsh, 1972, 1981~. Among the factors that have not yet been
studied systematically are expectations of success prior to mobility
training, family relationships, and living conditions.
Sensory and Motor Characteristics
Sensory and motor capacity are obvious contributors to the eventual
success or failure of mobility training. Because most of those who are
classified as visually impaired retain some degree of usable vision,
efforts have been to determine the residual capacity that characterizes
various visual impairments and to determine the capacities of the other
senses that either supplement remaining vision or replace lost vision.
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Marron and Bailey (1982) have shown that spatial contrast sensitivity
and visual fields are better correlates of mobility performance than
are standard measures of visual acuity. More research is needed to
expand existing knowledge of the relative contributions of residual
vision as well as hearing, touch, and gross motor ability to mobility
performance.
Perceptual and Cognitive Characteristics
Considerable attention has been paid to the perceptual and
cognitive factors on which mobility depends (Foulke, 1971, 1982b).
Barth and Foulke (1979) have pointed out the importance of preview, and
others have emphasized the importance of sensory training prior to
instruction (Mills and Adamshick, 19691. However, a preponderance of
the research relative to mobility has had as its objective a better
understanding of the effect of blindness on the ability to acquire and
use geographical concepts (Franks and Nolan, 1970, 1971; Lockman et
al., 1981; McR-ynolds and Worchel, 1954; Rieser et al., 1980; Worchel,
1951~.
There is a need for further study of the ability to acquire and use
information contained in echoes {Clark et al., 1975), the interactive
functioning of perceptual systems, and the processing mode that is best
suited for efficient performance of the mobility task.
MEASURES OF MOBILITY PERFORMANCE
In the past two decades, significant advances in the assessment of
mobility were made by researchers who developed objective measures of
overt behavior based on operationally defined criteria and observable
responses (Armstrong, 1975~. These methods, described below, have
provided rational and scientifically defensible tools for basic research
on mobility and for the objective evaluation of mobility aids and
training methods. Unfortunately, these objective measures have not
been used widely outside the laboratories in which they were developed.
In part, this appears to be due to inadequate documentation of the
methods and the need for specialized measuring equipment in order to to
employ them.
Indirect Measures
Indirect measures of mobility are those that assess the underlying
perceptual and cognitive processes that are responsible for overt
behavior. Such methods are used to enhance the sensitivity of
measurement by providing indices of stress or the demands placed on
mental capacity of the pedestrian performing the mobility task. Heart
rate measures obtained during mobility, for example, have provided
valuable data concerning the psychological stress experienced by the
blind traveler using different aids and performing in various
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environments (Heyes et al., 1976; Peake and Leonard, 1971; Tanaka et
al., 1981; Wycherley and Nicklin, 1970~. The secondary task technique
was used by Shingledecker (1983) to assess the reduction in the informa-
tion processing demands of travel as a function of route familiar ity
and previewed information. This technique involves having the
pedestrian engage in a secondary, unrelated task while negotiating a
route; it was also used to perform a performance benefits/mental costs
evaluation of mobility aids.
Despite the potential of these indirect measures of performance,
very little has been done in recent years to further their development.
However, several methods, such as analysis of the P300 component of
event-related brain potentials (ERP), have proven useful as direct
measures of the mental workload demanded by system design tasks and
might prove useful in basic mobility research and ETA evaluation as
well.
A majority of the performance assessment work done by mobility
specialists and many applied researchers is accomplished through the
use of subjective assessment methods. Unfortunately, these methods
have often consisted of informal and global expert judgments. Even
when simple rating scales and checklists have been used, examination of
subjective approaches has revealed evidence of rater bias and low
interjudge reliability (Airasian, 1973; Nye, 1973~.
Indirect measures correlate with direct measures of the mobility
task. That is, they are influenced by factors other than the com-
ponents of the mobility task and may include such measures as a
comparison of heart rates during travel in familiar and unfamiliar
environments or the personal assessment of confidence under varying
circumstances. Indirect measures of mobility may be further divided
into subjective and objective measures. Subjective measures include
ratings by subjects of their ability to perform a task and to recover
from errors and self-reports of mental workload. Objective measures
include stride length, secondary task performance, veering tendencies,
and the like.
Direct Measures
Direct measures used to assess mobility performance measure
specific aspects of mobility, such as obstacle detection or a mobility
specialist's subjective judgment of mobility skills. Examples of
direct, objective measures of mobility are found in Table 5. Direct
subjective measures include expert judgments (Airasian, 1973),
self-reports of the quality of performance, checklists and rating
scales (Genensky et al., 1979; Wurzburger, 1965), family reports, and
estimates of personal satisfaction (Airasian, 1973; Bailey et al.,
1984).
The direct observation and measurement of behavior often requires
expensive equipment and technical training and may prove impractical in
many settings outside the laboratory. A more practical approach in
these settings would be the use of instruments that rely on subjective
judgments, such as ratings scales, but instruments of this sort require
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TABLE 5 Examples of Direct, Objective Measures of Mobility
Safety
Efficiency
Travel Frequency
Travel Distance
Obstacle contacts
Unsafe veering
Fail to detect step-down
Fail to detect step-up
Inner shoreline body contact
Walking speed
Continuity of travel
Navigational errors
Veer
Route departures
Travel Familiarity or Difficulty
Device/Sensory System Specific Measures
Sign detection
Search time
Reading time
Surface structure/texture determination
Drop-off detection
Directional determination
Distance determination
Ratio of Independent to Sighted Guide Travel
Object or Landmark Detection
psychological scaling before they are suitable for use, and to date
little effort has been made to meet psychometric requirements.
Outcome Measures of Successful Mobility
In addition to objective measures of the performance of the mobility
task itself, a thorough assessment of orientation and mobility must
include measures of variations that are correlated with performance
variables (Darling et al., 1977; Gillman and Simon, 1980; Morrissette
et al., 1981~. These correlates are what might be called context
van ~ acres. They make up the background against which the mobility task
is performed. Included are such factors as social awareness and social
skills, perceptual and cognitive strategies, and spatial knowledge.
~ ~ . _ _
-
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Personal satisfaction with the quality of life may be the most
global and significant correlate of mobility, because mobility is a
prerequisite for so many of the activities that give meaning to life
(Blasch and Apple, 1975; Weisberger and Hall, 1985~. Few studies have
incorporated these issues systematically as design components.
ASSESSMENT OF MOBILITY
Assessment of Mobility Aids
Many of the travel aids that are currently or potentially useful to
blind and visually impaired pedestrians are based on the application of
electronics and optics technologies. In response to needs expressed by
the military service as well as the general public, we can expect both
of these technologies, including areas such as the study of artificial
intelligence, to make significant contributions to solutions to mobility
problems (Deering, 1985; Tou and Adjouadi, 1985~. The high cost of
travel aids that make use of new technology, no matter how effective
they might be, seriously limits their development. High cost may in
part be a consequence of the protection from competition afforded by
patents. For example, the Mowat Sensor, and the Sonicguide are both
patented inventions that can be produced only by authorized manu-
facturers. However, patents expire and other manufacturers may enter
the fields and may produce a greater variety of less expensive aids. A
recurring criticism of the way in which travel aids have been developed
is the lack of communication between their developers and such groups
as mobility specialists and specialists in the evaluation of per-
formance. The result of this failure of communication is that the
conception, development, and production of sensory aids is not guided
by full awareness of the needs of those who are expected to use them,
by a clear definition of their strengths and weaknesses, or by an
adequate understanding of the nature of the training their mastery will
require (Brabyn, 1985; Dodds, 1985; Dodds et al., 1983; Spungin, 1985;
Warren and Strelow, 1984b, 1985~.
A Standardized Method for Assessing Travel Performance
The field of mobility assessment is currently in a state of
disarray. There is an absence of controlled studies of mobility
skills. There is an incomplete understanding of the effects of
electronic travel aids, training methods, and other factors on
mobility. This situation is due in part to the fact that the field has
failed to adopt a standardized method for assessing travel performance.
One of the reasons for the lack of a common assessment method rests
in the fact that assessments are performed by a variety of individuals
to meet diverse goals. The basic research scientist, for example, may
use performance measures to test hypotheses about the nature of the
perceptual and cognitive processes that underlie travel behavior. The
applied research scientist might employ performance measures to evaluate
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the efficacy of a particular electronic travel aid. Finally, the
mobility specialist may use performance measures to evaluate a student's
progress, to formulate training goals, or to develop recommendations
for the prescription of a particular aid. The same assessment method
does not necessarily suit all situations. It is not really surprising,
therefore, that these professionals would select different measures to
achieve their particular objectives.
The differences among measures--such as those described in Table 5
and elsewhere in this chapter--can be appreciated by considering their
theoretical and practical dimensions. Preferred measures are those
that would be sensitive to changes in the individual's capabilities,
that can be reliably measured, and that validly index the skill being
assessed.
Performance assessment measures should provide information about
overt behavior, such as the quality of the travel skill displayed
overtly by the pedestrian. Such measures may be based on expert
judgments or on objective performance data that define safe, efficient,
and purposeful travel. These goals can be operationally defined in
terms of walking speed, obstacle contact, navigational accuracy, and
various productivity indices. Ideally, such measures should be
"device-free," that is, they should be applicable to an individual's
performance regardless of the travel aid used. However, some device-
specific measures may be justified if they are used to draw inferences
about some objective aspect of travel skill. For example, shoreline
cane contacts or efficient sign-reading with a telescope are valid
objective indices in an outdoor travel evaluation.
Performance assessment measures should also describe covert
processes that account for observed travel behavior, such as the
psychological cost of mobility under different travel conditions or in
the use of various travel aids. Such information may be obtained from
subjective reports, behavioral observations, or electrophysiological
measurements. Historically, such measures have included rating scales
to assess travel ease and confidence, heart rate parameters to assess
stress levels, and secondary tasks Performed in conjunction with
_ _ _
mon~ty to assess the reserve information-processing capacity of the
traveler.
An important feature of indirect measurement is its ability to
increase the overall sensitivity of the assessment procedure by
detecting differences that are not reflected in general measures of
overt performance. For example, two travel aids may provide equivalent
overt performance but, in one case, the pedestrian is performing at the
limits of his or her information-processing capacity. It is important
to realize that direct and indirect measures are not mutually exclusive
alternatives in the assessment of mobility performance.
Finally, an inescapable fact regarding the assessment of mobility
is the necessity to isolate the individual components of mobility skill
and to synthesize them once more into a global evaluation of total
performance. From a practitioner's point of view, the emphasis must be
placed on whole-skill measurement.
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RECOMMENDATIONS
It is clear from the information presented in this chapter that
there is a need to develop a framework or theory of mobility that
identifies the perceptual and cognitive components of mobility and that
makes explicit the extent to which mobility is influenced by such
factors as training, travel aids, the nature of the environment in
which the mobility task is performed, and social variables. We need to
develop consistent techniques and measures of mobility performance so
that experiments can be replicated and their results compared. Finally,
there is a need for improved experimental designs, which reflect more
careful attention to such issues as the use of experimental controls,
sample size, statistical analysis, and the generalizability of the
results of experiments. In order to attain these goals, we make the
recommendations that follow.
Standardized Measures of Mobility Performance
Accurate and practical assessment of the performance of blind and
visually impaired pedestrians is urgently needed. It is not likely
that a single scheme that is appropriate to all situations can be
developed, but the development of several coordinated methods is a
realistic objective. Standardized procedure for evaluating mobility
aids and a standardized procedure for assessing and comparing the
performances of subjects with different kinds and degrees of visual
impairment would be immediately useful to researchers and practitioners
alike. Explication of these procedures should include statements of
the rationale for all of the instruments involved in assessment, and
complete documentation should be provided for each instrument.
There is also a need to develop additional indirect measures of
mobility performance. These might include measures of the performance
of a secondary task, such as the task of conducting a conversation with
a companion while traveling on foot independently. Because fluctuations
in the performance of a secondary task often permit inferences con-
cerning relatively subtle changes in the performance of the primary
task, important differences in task performance can be detected only by
indirect measures.
RECOMMENDATION: Efforts to develop improved methods for measuring
performance of the mobility task should be a research priority for
federal funding agencies involved in this area. Research should be
undertaken immediately on the development of improved objective
measures of mobility performance, drawing on past efforts in England,
the United States, and New Zealand.
Simulation Methods
There is a need to develop simulation environments to study
mobility performance and its assessment. Such simulations, while
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preserving fidelity to the real world in important respects, permit
experimental controls that cannot be enforced in the real world. We
will not make significant progress in assessing the performance of
blind travelers, evaluating training methods and travel aids, and
designing improved travel aids until our efforts are guided by a
comprehensive theory of mobility that has been confirmed by the results
of numerous experiments, and many of these experiments can be conducted
more effectively and efficiently in a simulated environment than in any
other setting. However, the construction of simulated environments is
an expensive undertaking, for which federal assistance is not likely.
Simulated spaces must be large enough to permit realistic performance
of mobility tasks. Such spaces should also have the physical and
acoustical characteristics of the surfaces and space an individual will
encounter in the real world (Shingledecker and Foulke, 1978~.
RECOMMENDATION: Investigators should more frequently use simulated
environments to study mobility performance and its assessment. The
private sector could contribute to the development of simulation
methods through the development of simulation technology.
Improved Field Assessment Techniques
Assessment techniques used in the laboratory often require expensive
instruments and facilities and technically trained personnel. Conse-
quently, their use in field studies is not practical. More subjective
techniques such as rating scales, expert judgments, and the like, can
be made serviceable if they are validated by the application of psycho-
metric methods; this work is yet to be done.
RECOMMENDATION: Improved field assessment techniques should be
developed through strengthened support for research in which psycho-
metric methods are used to develop rating scales and other measuring
instruments of that type.
Technology Benchmarks
As developments in technology continue, new travel aids, which go
beyond the current state of technology, will be built. As a necessary
step in their development, a rigorous comparison will be needed of the
performance facilitated by travel aids now in use. The information
yielded by such comparisons is essential in deciding about the actual
contribution made by new travel aids to the performance of the mobility
taske
RECOMMENDATION: Because progress in the development of travel aids
and better methods for their assessment will depend in part on making
individuals in many different disciplines more aware of the current
state of mobility problems, we recommend that communication among such
groups as engineers, psychologists, mobility specialists, teachers, and
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consumers be facilitated. Federal funding agencies can play a role by
actively funding conferences and workshops to further the development
of performance comparisons of new and existing technologies.
Training Programs
It is not sufficient to assess the improvement in performance made
possible by the use of a travel aid. We must also do a more effective
job of assessing the improvement in performance that is attributed to
participation in a training program.
Furthermore, we must do a more
conscientious job of evaluating the programs in which blind pedestrians
are trained and the programs in which mobility specialists are trained.
RECOMMENDATION: We recommend that academic departments in which
mobility specialists are trained and the federal agencies from which
they receive support cooperate in providing more training for students
in the assessment of performance.
Representative terms from entire chapter:
travel aids
