• investigate, test, and incorporate conceptual developments; and

• develop, pretest, pilot test, and revise measurement instruments and design.

In conclusion, the immediate need of the NSHA involves estimates of the size and characteristics of the pool of persons eligible for SSA disability benefits. A cross-sectional sample of the household population done at a particular point in time provides useful estimates for such needs. When change over time is an issue, survey measurements must be repeated in order to provide estimates of change. When the only interest is whether the full target population has experienced a change in the prevalence of a phenomenon, an independent cross-sectional survey conducted at a later time provides useful change estimates. When the interest concerns whether some types of individuals change and others do not, a longitudinal survey, conducting repeated interviews of the same persons, provides the most useful data.

SSA’s needs for the estimation of change over time in the size and characteristics of the eligible population stem from the necessity to forecast the growth or decline of the applicant and beneficiary pool. SSA has stated that NSHA will permit forecasting of changes in the size of the beneficiary population. Such a goal implies ongoing measurement of the size and characteristics of the eligible population, with updated instrumentation to reflect any changes in conceptual and measurement issues and in SSA’s eligibility protocol that may have occurred in the intervening years.

The next chapter discusses the design choices for obtaining the needed information on an ongoing basis using a reduced set of measures in the intervening years between the conduct of the large surveys.