Fast Calculation of Calendar Time-, Age- and Duration Dependent Time at   Risk in the Lexis Space
  In epidemiology, the person-years method is broadly used to estimate the incidence rates of health related events. This needs determination of time at risk stratified by period, age and sometimes by duration of disease or exposition. The article describes a fast method for calculating the time at risk in two- or three-dimensional Lexis diagrams based on Siddon's algorithm. 
Ralph Brinks
stat.ME stat.CO