Two longitudinal studies, the SIPP and the PSID, that in many respects appeared to be good candidates for enhancement, were eventually considered to have samples that were too small to provide reliable estimates of recently homeless or residentially unstable families.
The SIPP is a series of national panel studies designed to collect information on income, labor force participation, and participation and eligibility for various government programs. The length of time each panel is followed has varied in recent years, from 2.5 to 4 years. Sample sizes have also varied from cohort to cohort within the panel studies, from 14,000 to 36,700 households in the 2001 study. Even at its largest, however, the SIPP study is likely to identify only 500 or 600 recently homeless families at most (based on a 1.5% annual homeless rate). Although this would be a sufficiently large sample to examine national trends, it would not provide a large enough sample to reliably examine any regional or geographic differences in homelessness. Combined with the fact that the SIPP tracks families for only a few years, it does not appear to be a good candidate for enhancement.
Initially, the PSID offered the best prospects for informing national efforts toward homeless prevention and resource allocation. Begun in 1968 and conducted by the University of Michigan, the PSID originally consisted of two independent samples-a cross-sectional national sample of approximately 3,000 families and a national sample of 2,000 low-income families. From 1968 to 1996, individuals from these initial samples were interviewed annually, including people who may no longer have been living in the original sampled household (e.g., children of the originally selected households). Because it tracked everyone associated with the originally sampled household, by 1996 the PSID had grown to over 65,000 individuals. In order to keep the sample more manageable, as well as to readjust the sample to better reflect the U.S. population, adjustments were made to the sample in 1997 that reduced the number of "core" families and added a new sample of families, particularly Latino and Asian households. The distinct advantages of the PSID with respect to being able to address knowledge gaps about homeless families are the following:
- Longitudinal, currently conducted every 2 years;
- Long history, starting in 1968;
- An oversample of low-income households, who have a higher probability of having been or becoming homeless than the general population;
- A residential followback as part of its data collection, so many of the changes could be adding questions to that part of the instrument; and
- A wealth of data that have been consistently collected over time, such as income sources and amounts, employment, family composition, and demographic changes.
The major limitation of the PSID, however, is its sample size. If 1.5 percent of the households in the current PSID sample experienced homelessness in any given year, this would produce a sample of only 75 families to examine given the current overall sample of 4,800 households. A further complication with the PSID, or with any longitudinal study, is the ability to track and maintain contact with more difficult-to-reach study participants, such as people or families who become homeless. The response rates for the PSID have generally been very high, averaging 97 percent to 98 percent a year.10 As is noted in the PSID guide, though, even small rates of attrition from wave to wave can create problems over time. In 1988, for example, the response rate for individuals who lived in 1968 households was only 56 percent. Furthermore, the PSID does not make an attempt to recontact households that drop out, so even a small level of attrition may severely impact the likelihood of identifying families that have been or become homeless. Thus, despite its many potential advantages, concerns over sample size and composition make the PSID a less than ideal candidate for enhancement.
The only two longitudinal surveys that do seem to have some potential for addressing knowledge gaps about homeless families are the two recent NLS cohorts: the National Longitudinal Surveys of Youth 1979 (NLSY79) and the National Longitudinal Surveys of Youth 1997 (NLSY97). Both the NLSY79 and the NLSY97 are part of the National Longitudinal Surveys conducted for the U.S. Department of Labor, BLS.