Adjustment of 1979-1993 NHSDA Estimates to Account for the New Survey Methodology Used in the 1994 and Later NHSDAs

The NHSDA is an important source of data for policymakers, not only because it provides measures of substance abuse for a single year, but also because the series of surveys over the past several years provides a measure of change in substance abuse in the population over time. Beginning in 1994, the NHSDA began using an improved questionnaire and estimation procedure based on a series of studies and consultations with drug survey experts and data users. Because this new methodology produces estimates that are not directly comparable to previous estimates, the 1979-1993 NHSDA estimates presented in Chapter 2 of this report were adjusted to account for the new methodology that was begun in 1994.

Nearly all of the 1979-1993 substance use prevalence estimates presented in Chapter 2 were adjusted using a simple ratio correction factor that was estimated at the total population level using data from the pooled 1993 and 1994 NHSDAs. The remaining substance use prevalence estimates were adjusted by formally modeling the effect of the new methodology, relative to the old methodology, using data from the 1994 NHSDA. The modeling procedure was used for the more prevalent substance use measures that changed significantly between the old- and new-version NHSDA questionnaires. The modeling procedure was particularly desirable for the more prevalent measures because the procedure was able to use a greater number of potentially significant explanatory variables in the adjustment compared with the simple ratio correction factor. Each of the procedures is discussed below.

This page was last updated on December 30, 2008.