scribed flows as well as unprescribed flows, i.e., you can back up and go pretty much anywhere you want.
And that ability to back up and move forward and back up creates an infinite number of unique instruments for any given instrument that we can field. Obviously we don’t implement any more than we have respondents to talk to, but we have the capacity to have huge numbers of different variations on the instrument. And the important thing is that all of these have to work perfectly, because we don’t know until we get into the field which unique path has to be functioning for a given observation.
The Census Bureau is known for doing complex surveys. We have always done them. We did challenging surveys in paper; we’ve done challenging surveys in computer-assisted telephone interviewing (CATI). But, boy, now that we’re in computer-assisted personal interviewing we really have some complex instruments, one of which is the Survey of Income and Program Participation (SIPP). [For illustrative purposes, a one-page excerpt from a pen-and-paper version of the SIPP instrument is shown in Figure II-1.]
SIPP is a longitudinal survey, which means that we go back to the same set of respondents repeatedly. It’s nine rounds of interviewing for each sample that we have in the survey. We do three interviews a year; we go back every four months to the same people and also talk to whomever else they live with. We spread the interviewing out over time to equalize the field work load, so every month we’re doing about one quarter of the sample.3 We have an instrument that has about 13,000 different items in it that we’ve put out in the field, that we’ve automated. It’s personal interviewing—carry the laptop to the house, or be at home with the laptop making telephone calls. The instrument is completely coded on the laptop, and there [are] some separate systems that control when the instrument comes in and when the case is set up and so forth.
The information that we collect is monthly, so for every month of the four-month reference period we collect information. We have data that we collect every wave, which is what we call “core information.” We have that for income—well over 50 different income sources. Extensive demographic characteristics. A lot of work history and activities, so we
The sample of households to which SIPP is administered is known as a “panel.” To divide up the workload, the group is broken into four subsamples; one of these “rotation groups” or “rotations” is interviewed each month, hence each panel member is interviewed every four months. At that interview, the respondent is asked to report based on the preceding four months; this four-month window is the “reference period” for the survey. Each round of interviewing for a particular household is known as a “wave,” so that the first time the interviewer contacts the household is called Wave 1, the second contact Wave 2, and so forth. For additional detail on the SIPP and its sample design, see Westat (2001) and National Research Council (1993).