Calculates occurrence / exposure rates for time periods given by
period and for ages given by age.
perstat(surv, period, age = c(0, 200))An (extended) surv object (4 columns with
enter, exit, event, birthdate)
A vector of dates (in decimal form)
A vector of length 2; lowest and highest age
A list with components
No. of events in eavh time period.
Exposure times in each period.
events / exposure