Income Data for Policy Analysis: A Comparative Assessment of Eight Surveys. Rolling Samples

12/23/2008

Both the ACS and the NHIS utilize a rolling sample. In each case, an annual sample is distributed systematically over the year. For the ACS, with an annual sample of 3 million households when fully implemented, distributing the workload over the year is an operational necessity. For the NHIS, operational considerations may be important as well, but another factor in the design is the seasonality and possible trend in some of the health measures that the survey collects. Rolling samples raise questions about the best approach to measuring characteristics that can vary over time, and the ACS and NHIS illustrate two different approaches to measuring annual income. The ACS asks respondents to report their income for the past 12 months, which is defined as “the period from today’s date one year ago up through today.”52  This represents a rolling reference period with a non-varying recall interval. The NHIS asks respondents, regardless of when they are interviewed, to report their incomes for the previous calendar year. This yields a fixed reference period but with a varying recall interval. Choices such as this one carry implications for the interpretation of estimates and may ultimately affect the quality of the data collected. In this case, is one choice clearly better than the other?  In attempting to answer this question, we begin by examining some of the issues raised by the use of rolling samples to collect data for policy analysis. We then turn to empirical analyses bearing, first, on rolling reference periods and, then, on varying recall intervals. We conclude this discussion by looking at the within-year inflation adjustments developed for the ACS in order to put the data collected with a rolling reference period into the same real dollars across the different 12-month intervals used as reference periods.

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