/* DATE 4\24\91                        */
/*   CUMMULATIVE POISSON PROBABILITY
     Joseph Hilbe - STB             */

program define cpoisson
version 2.1
if "%_3"=="" {
  di in re "Type 3 arguments:"
  di in re "     pop ratio  observations   cases"
  exit 198
}
drop _all
quietly  {
  set obs %_3
  gen double expect=%_1*%_2
  gen double prob0=exp(-expect)
  gen double cum=0
  gen double prob=prob0
  gen num=[_n-1]+1
  if (%_3>0)   {
    while num<_N {
      replace prob=prob*expect/num
      replace num=num+1
      replace cum=cum+prob  
    }
    replace cum=cum+prob0
    replace prob=1-cum
  }
  if (%_3<1) {
     set obs 1
     gen unit=1.0
     replace prob=unit
     }
di in gr "Cumulative Poisson Probability => " in ye prob
drop _all
end