ARFAR: a person years at risk program.

Cohort studies are often used to compare the health experience of groups of people. Several methods of analysis exist, such as the use of standardised or proportional mortality rates' and the proportional hazards model.2 The calculation of a standardised mortality ratio (SMR) needs to take into account the effect of confounding factors, such as age and sex, which may affect mortality rates. The use of person years at risk to calculate an SMR is a means of controlling for these factors.3