An Epidemiologic Note on Verification Bias: Implications for Estimation of Rates.

Two-stage designs offer several advantages for purposes of test validation and prevalence estimation. These include enhanced precision and increased cost-efficiency. Cost efficiency is obtained when the best available verification criterion is too expensive to employ in a large-scale epidemiologic study. The use of two-stage (or double-sampling) designs permit the same inferences from application to only a subset of those who were screened (with the less expensive criterion) during stage one. The retesting of only some of the first stage respondents introduces a bias, however, if these are sampled on the basis of first stage screening results. The form of this bias is described and solutions for correcting estimates are provided. These solutions are applied to the data reported by Abbott and Volberg (1996) in their study of the New Zealand general population. Corrected estimates of the sensitivity and specificity of the South Oaks Gambling Screen are obtained and used to adjust the reported lifetime and current prevalence estimates. The value of multi-stage designs for validity assessment and prevalence estimation are briefly described.