In most cohort studies it is necessary to calculate the actual time-at-risk for each individual as person-time. The person-time is used to either calculate mortality or incidence rates of the cohort or to perform indirect standardization or multivariate analysis. Person-time was calculated in person-years (PY) by a SAS® macro. The macro uses the three time variables of age, length of stay in Germany and calendar-year. The macro calculates and distributes the person-years exactly to the day. Age and length of stay are categorized in one year intervals. Afterwards age is categorized into five year age groups up to 85 and older
