Correct bfmi denominator (arviz-devs#991)

* correct bfmi denominator * update test_diagnostics.py * fixed denominator and docs
percygautam · Jan 14, 2020 · 18a1dc4 · 18a1dc4
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diff --git a/arviz/stats/diagnostics.py b/arviz/stats/diagnostics.py
@@ -46,6 +46,7 @@ def bfmi(data):
     Examples
     --------
     Compute the BFMI of an InferenceData object
+
     .. ipython::
 
         In [1]: import arviz as az
@@ -526,9 +527,9 @@ def _bfmi(energy):
     energy_mat = np.atleast_2d(energy)
     num = np.square(np.diff(energy_mat, axis=1)).mean(axis=1)  # pylint: disable=no-member
     if energy_mat.ndim == 2:
-        den = _numba_var(svar, np.var, energy_mat, axis=1, ddof=0)
+        den = _numba_var(svar, np.var, energy_mat, axis=1, ddof=1)
     else:
-        den = np.var(energy, axis=1)
+        den = np.var(energy, axis=1, ddof=1)
     return num / den
 
 

diff --git a/arviz/tests/test_diagnostics.py b/arviz/tests/test_diagnostics.py
@@ -45,7 +45,7 @@ def data():
 class TestDiagnostics:
     def test_bfmi(self):
         energy = np.array([1, 2, 3, 4])
-        assert_almost_equal(bfmi(energy), 0.8)
+        assert_almost_equal(bfmi(energy), 0.6)
 
     def test_bfmi_dataset(self):
         data = load_arviz_data("centered_eight")