Causation is defined by a different risk in the entire population under two potential exposure values; association is defined by a different risk in the subsets of the population determined by the subjects’ actual exposure value. Causation is defined by a different risk in the entire population under two potential exposure values; association is defined by a different risk in the subsets of the population determined by the subjects’ actual exposure value. M A Hernán J Epidemiol Community Health 2004;58:265-271 ©2004 by BMJ Publishing Group Ltd