Lay persons' understanding of the risk of Down's syndrome in genetic counselling.

Genetic counselling traditionally expresses risk in proportions (e.g. 1 in 112) rather than as rates (e.g., 8.9 per 1,000). The justification for this practice is unclear. To assess the understanding of lay persons of the risk of Down's Syndrome, whether expressed as rates or as proportions, we analysed 589 self-administered questionnaires. Overall, respondents understood rates significantly better than proportions (76.2% vs 72.3% correct, respectively; P = 0.03) Evidence from two studies in disparate populations suggests that rates are better understood and thus are the preferred way to explain genetic risk to lay persons.