Quantification of the dependence of the odds ratio comcomitant variables associated with each of several 2 .times. 2 tables, using a regression model proposed by Zelen, is an important tool for retrospective studies in epidemiology. An exact analysis may be based on the conditional likelihood obtained by fixing all the marginal totals. Asymptotically this approach is equivalent to use of an unconditional log-linear model. The method is used to reanalyze data reported by Kneale on the relationship between obstetric radiation and childhood cancer.