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+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