A 2-STAGE VALIDATION-STUDY FOR DETERMINING SENSITIVITY AND SPECIFICITY

A two-stage procedure for estimating sensitivity and specificity is de scribed. The procedure is developed in the context of a validation stu dy for self-reported atypical nevi, a potentially useful measure in th e study of risk factors for malignant melanoma. The first stage consis ts of a sample of N individuals classified only by the test measure. T he second stage is a subsample of size m, stratified according the inf ormation collected in the first stage, in which the presence of atypic al nevi is determined by clinical examination. Using missing data meth ods for contingency tables, maximum likelihood estimators for the join t distribution of the test measure and the ''gold standard'' clinical evaluation are presented, along with efficient estimators for the sens itivity and specificity. Asymptotic coefficients of variation are comp uted to compare alternative sampling strategies for the second stage.