tion prevents the person from working or limits the kind or amount of work. Once again, note the lack of distinction between the ability to perform the activities associated with the actual performance of the job and those activities related to the role of work. For those who retire early because of a health condition or impairment, would the respondent consider that health problem as keeping the person from working?
The point of the examples presented above is not to criticize the questionnaires in which they appear but rather to illustrate the problem of attempting to measure a complex, multidimensional, dynamic construct with a single question or a set of two questions. No one or even two questions can possibly tap into the various components of work disabilities. Clearly the first step toward a robust set of screening items is the acceptance of a shared conceptual framework and understanding of the dimensions of the construct of interest. That framework must consider the social environment in which the measurement of interest will be taken, understanding that the comprehension of the question is shaped not only by the specific words used in the question and the context of the question, but by the perceived intent of the question. The use of cognitive laboratory techniques can aid in the identification of problems of comprehension due to the use of inherently vague terms and differential perceptions of the intent of the question. Such techniques will aid in the understanding of the validity of the questions and, through the refinement of the wording of questions, hopefully improve the reliability of the items.
Simply documenting that variation in the essential survey conditions of the measurement process contributes to different estimates of persons with work disabilities is not sufficient; the marginal effects of various factors need to be measured and the impact needs to be reduced through the use of alternative design features. Both of these can be accomplished only through a program of experimentation. Similarly, the psychometric properties of these measures need to be assessed. Without undertaking a thorough program of development and evaluation, the discrepant estimates evident in the empirical literature will persist.