Merge pull request scipy#371 from rgommers/ticket-1779-nbinom-pmf

BUG: fix precision issue with nbinom.pmf.  Closes ticket 1779.

Author: Ralf Gommers

scipy/stats/distributions.py

@@ -6529,8 +6529,7 @@ def _rvs(self, n, p):
     def _argcheck(self, n, p):
         return (n >= 0) & (p >= 0) & (p <= 1)
     def _pmf(self, x, n, p):
-        coeff = exp(gamln(n+x) - gamln(x+1) - gamln(n))
-        return coeff * power(p,n) * power(1-p,x)
+        return exp(self._logpmf(x, n, p))
     def _logpmf(self, x, n, p):
         coeff = gamln(n+x) - gamln(x+1) - gamln(n)
         return coeff + n*log(p) + x*log(1-p)

scipy/stats/tests/test_distributions.py

@@ -160,6 +160,12 @@ def test_rvs(self):
         assert_(isinstance(val, numpy.ndarray))
         assert_(val.dtype.char in typecodes['AllInteger'])
 
+    def test_pmf(self):
+        # regression test for ticket 1779
+        assert_allclose(np.exp(stats.nbinom.logpmf(700, 721, 0.52)),
+                        stats.nbinom.pmf(700, 721, 0.52))
+
+
 class TestGeom(TestCase):
     def test_rvs(self):
         vals = stats.geom.rvs(0.75, size=(2, 50))