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 1000). 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 analy sed 589 self-administered questionnaires. Overall, respondents understood r ates significantly better than proportions (76.2% vs 72.3% correct, respect ively; P = 0.03) Evidence from two studies in disparate populations suggest s that rates are better understood and thus are the preferred way to explai n genetic risk to lay persons.