ASSESSING BIAS IN COMMUNITY-BASED PREVALENCE ESTIMATES - TOWARDS AN UNDUPLICATED COUNT OF PROBLEM DRINKERS AND DRUG-USERS

General population survey estimates of the overall prevalence of probl em drinking and drug use in a community are biased by the exclusion of non-household populations. Estimates based on compiling prevalences i n community institutions may also be biased due to over-counting of us ers of more than one institution. This paper examines prevalence estia mtes derived from probability samples of problem drinkers in the gener al population and within alcohol treatment, drug treatment, mental hea lth, criminal justice and welfare agencies in a single US county. Data sets are merged and weighted to reflect a community sample of institu tions, and a 17% subset of cases is identified within the institutiona l samples that are not living in housing units typically included in g eneral population sampling frames. The difference in prevalences of pr oblem drinking in the household and non-household populations is found to be large: 11% and 48%, respectively. Even greater differences are found between estimates of unprescribed weekly drug use (6% and 47%, r espectively) and combined problem drinking and weekly drug use (2% and 27%, respectively). This suggests that confining samples to the house hold population can systematically under-represent the prevalence of p roblem drinking and drug use. A second source of bias in prevalences i s characteristic of studies using records from multiple institutions. When duplication of service use in the five agency samples is consider ed, it becomes apparent that prevalences may be biased upward due to o ver-counting of multiple service users.