On the basis of functional limitations, the SIPP estimated that 39.5 million people, 17 percent of the noninstitutionalized population, had some degree of disability. (In Figure 2-3, this number is the sum of all numbers in boxes labeled "limitations in functioning," i.e., 2,326 + 9,677 + 8,422 + 3,443 + 118 + 11,310 + 4,157). Another example is work limitations, where the number of persons in the population ages 18-64 with any work limitation totals 17.95 million people (i.e., 8,442 + 3,443 + 549 + 5,515).

Table 2-1 shows that the SIPP produced a lower estimate of disability among children than the NHIS. Some of the difference may be due to different types of questions. The SIPP asks a general question on whether any children have limitations, and only with a positive response does it go on to ask which children, up to a total of three. The NHIS includes an individualized inquiry on the presence of activity limitation for each child in the household. For younger adults, the two surveys produce comparable results. At older ages, however, the SIPP shows much higher rates of disability than the NHIS. For people 65 and over, the SIPP rate is half again as high as the NHIS rate. The more extensive questions in the SIPP on the ability to perform specific functions may provide a greater opportunity for respondents to identify limitations.