The patient year (or person year) statistic is used in many clinical studies and statistical assessments of risk.
Patient years are calculated as follows: If 15 patients participated in a study on heart attacks for 20 years, the study would have involved 300 patient years (15 x 20). This number can be divided by the number of patients who have been affected by a certain condition or event. For example, if six of the patients had heart attacks, that would be equal to one heart attack for every 50 patient years in the study (300 / 6 = 50).
Looking at data in this way can reveal trends and allows researchers to communicate levels of risk. Many studies on new medicines express their findings using patient years. For example, if one serious side effect is experienced for every 1,000 patient years of a study, this might be considered an acceptable level of risk.