Ratio Adjustment

Most of the 1979-1993 NHSDA estimates were adjusted using a ratio correction factor that measured the effect of the new methodology, relative to the old methodology, using data from the 1993 and 1994 NHSDAs. As explained in the introduction to Advance Report No. 18 (OAS, 1996b), the 1994 NHSDA was designed to generate two sets of estimates. The first set of estimates, which in previous reports was referred to as the 1994-A set of estimates, was based on the same questionnaire and editing method that was used in 1993 (and earlier). The second set of estimates, referred to as the 1994-B set, was based on the new NHSDA survey methodology. Because the 1994-A estimates were generated from a sample that was roughly one-fourth the size of the 1994-B, to increase the precision of the ratio correction factor, the 1994-A sample was pooled with the 1993 sample.

The 1979-1993 NHSDA estimates that were adjusted using the ratio correction factor included estimates of lifetime, past year, and past month use of cocaine, crack, inhalants, hallucinogens (including PCP and LSD), heroin, any psychotherapeutic, stimulants, sedatives, tranquilizers, analgesics, any illicit drug other than marijuana, and smokeless tobacco, as well as estimates of past year frequency of use of marijuana, cocaine, and alcohol. This adjustment was computed at the total sample level and was applied equally to all corresponding estimates computed among subgroups of the total population. Consequently, for example, the same ratio adjustment was used to correct all estimates of past year cocaine use, regardless of the demographic subgroup under consideration. Mathematically, this ratio adjustment can be expressed as follows: Suppose i denotes the sampled respondent, y_i denotes a 0/1 variable to indicate nonuse or use of some particular substance, and w_i denotes the sample weight. Then the ratio adjustment was computed as

The latter equality is true because the sample weights in the pooled 1993 and 1994-A sample were adjusted slightly so that they would sum to the same demographic control totals as the 1994-B sample across the variables typically used in the NHSDA poststratification procedure.

This page was last updated on December 30, 2008.