A model for media exposure probabilities is developed which has the joint probability of exposure proportional to the product of the marginal probabilities. The model is a generalization of Goodhardt & Ehrenberg's ‘duplication of viewing law’, with the duplication constant computed from a truncated canonical expansion of the joint exposure probability. The proposed model is compared on the basis of estimation accuracy and computation speed with an accurate and quick ‘approximate’ log-linear model (as noted previously)and the popular Metheringham beta-binomial model. Our model is shown to be more accurate than the approximate log-linear model and four times faster. In addition, it is much more accurate than Metheringham's model.