MAINT: bradford distribution accuracy

oavdeev · Jun 10, 2016 · 9c513f7 · 9c513f7
1 parent a21d69c
commit 9c513f7
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Showing 2 changed files with 13 additions and 3 deletions.
diff --git a/scipy/stats/_continuous_distns.py b/scipy/stats/_continuous_distns.py
@@ -581,13 +581,13 @@ class bradford_gen(rv_continuous):
 
     """
     def _pdf(self, x, c):
-        return c / (c*x + 1.0) / log(1.0+c)
+        return c / (c*x + 1.0) / special.log1p(c)
 
     def _cdf(self, x, c):
-        return log(1.0+c*x) / log(c+1.0)
+        return special.log1p(c * x) / special.log1p(c)
 
     def _ppf(self, q, c):
-        return ((1.0+c)**q-1)/c
+        return special.expm1(q * special.log1p(c)) / c
 
     def _stats(self, c, moments='mv'):
         k = log(1.0+c)

diff --git a/scipy/stats/tests/test_distributions.py b/scipy/stats/tests/test_distributions.py
@@ -224,6 +224,16 @@ def test_entropy(self):
         assert_equal(h, 0.0)
 
 
+class TestBradford(TestCase):
+    # gh-6216
+    def test_cdf_ppf(self):
+        c = 0.1
+        x = np.logspace(-20, -4)
+        q = stats.bradford.cdf(x, c)
+        xx = stats.bradford.ppf(q, c)
+        assert_allclose(x, xx)
+
+
 class TestNBinom(TestCase):
     def test_rvs(self):
         vals = stats.nbinom.rvs(10, 0.75, size=(2, 50))