Assessing comorbidity through prevalence surveys: problems and pitfalls

Lynn S. Wallisch, Ph.D.

States and other jurisdictions routinely carry out epidemiological or needs assessment surveys to plan for substance abuse, mental health and gambling treatment services. Multi-focus surveys that ask about more than one of these behaviors allow for estimates of comorbidity as well as being more cost efficient, but can lead to serious biases. The present study examines some apparently inconsistent survey findings that do not appear to reflect true differences in behavior, and discusses the possible unexpected sources of bias. Four telephone surveys carried out by the state of Texas between 1993 and 1996 are compared: two primarily focused on gambling (1992 and 1995) and two on substance use (1993 and 1996). All surveys were designed to represent the general population of adults living in households in Texas, with sample sizes between 6300 and 8000. Overall survey design and methodology, and specific questions about gambling and substance use prevalence and problems, were similar (though not identical) across surveys. A comparable question about mental health was also asked in three of the surveys. It was discovered that gambling prevalence and problem gambling were underreported on the substance-focused surveys, while substance use prevalence was underreported on the gambling-focused surveys. Substance dependence, however, was reported at about the same level on all the surveys. Mental health problems were also underreported in the gambling surveys as compared to the substance survey. Subtle differences among the surveys, such as question placement and context, amount of detail requested, and survey focus are discussed as factors that may have contributed to discrepant results, and implications for survey instrument development are considered.

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Addiction Research Institute

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