Guralnik observed that the Nagi theoretical model of the pathway from disease to disability has been very helpful in terms of operationalizing the assessment of steps along the pathway and particularly useful in thinking about where performance measures fit. Certainly, objective measures of performance can be done at several of the steps in the model—impairment, functional limitation, and disability. Impairments objectively measure physiologic functioning. At the final step, disability, one may be observing people in standardized home-type environments. However, performance measures, such as gait speed, chair rises, and pegboard tests, have been used mostly in the domain of functional limitations.
Guralnik offered three performance assessments to illustrate the points made in his presentation: gait speed, the index of mobility-related physical limitations (MOBLI) developed by Lan and Melzer (Lan et al., 2002), and the Short Physical Performance Battery (SPPB).
MOBLI was developed using data from the National Health and Nutrition Examination Survey III (NHANES III), empirically looking at measures that were related to mobility, which include gait speed, chair rises, and a pulmonary function test. The index was then validated in other studies. The components of SPPB include timed standing balance, a timed 4-meter walk, and timed multiple chair rises. This battery was first developed in the Established Populations for Epidemiologic Studies of the Elderly (EPESE) in 1988. SPPB has very good psychometric properties: It predicts mortality, nursing home admission, new disability, and health care expenditures, among other things, and it has good reproducibility. It is sensitive to clinically important change.
One of the issues related to performance testing in general is that the scoring of some tests does not have a way of dealing with people who are unable to perform the task. So it is difficult to know how to handle people who are unable to perform the test. For example, if gait speed is used, what do you do if someone just cannot walk at all? People have approached this issue in different ways, but it is a limitation of performance measures that researchers rarely address. Even in determining why a test was not done, people often fuss with the data—trying to understand if the data are missing because the person really was not able to do the test and so should be scored as a 0 or given a poor score or whether the person simply refused.
Sometimes even refusals can be vague. People refuse because they are afraid to do the test because they know that they are going to be unable to do it. Sometimes the responsibility for a refusal is placed on the examiner, which is a bit unfair, but it is sometimes hard to sort out when the researcher does not know what the data on the performance test mean. One solution to this problem, used in the SPPB, is to create categorical scores that cover the range of functioning and give a 0 score to those unable to do the test.