According to data from the WHO Multinational Monitoring of Trends and Determinants in Cardiovascular Disease (MONICA) project,24 the ratio of incident myocardial infarction and coronary death in Northern Ireland (Belfast) compared with in France (Toulouse, Strasbourg, and Lille) was between 2:1 and 3:1.We used results of the MONICA project to calculate the sample size for a country specific analysis of ischaemic heart disease in PRIME, which used the same populations. Observation of 100 cases within each country cohort would permit detection of a hazard ratio of about 1.8 (with risks α=0.05 and β=0.20), associated with an exposure factor prevalence of 0.5. To allow identification of this number of events, a sample size of 2500 men in Northern Ireland and 7500 in France observed over a five year period was designated a priori. Lower than expected numbers of cases were observed in PRIME, however, owing to the important reduction in the rates of coronary heart disease during the 90s, especially in Northern Ireland. It was thus decided to extend follow-up to 10 years for many risk factors analysed in PRIME.
