text: c function ppoiss.f
  c
  c source
  c   Bevington, page 39.
  c
  c purpose
  c   evaluate poisson probability function
  c
  c usage
  c   result = ppoiss (nobs, averag)
  c
  c description of parameters
  c   nobs   - number of items observed
  c   averag - mean of distribution
  c
  c subroutines and function subprograms required
  c   factor (n)
  c      calculates n factorial for integers
  c
  	function ppoiss (nobs,averag)
  1	ppoiss=((averag**nobs)/factor(nobs))*exp(-averag)
  	return
  	end
  