Add failing test t_est_missing_formula

statsmodels/genmod/tests/test_gee.py

@@ -133,6 +133,39 @@ def test_missing(self):
         assert_almost_equal(rslt1.params, rslt2.params)
         assert_almost_equal(rslt1.bse, rslt2.bse)
 
+    def t_est_missing_formula(self):
+        """
+        Test missing data handling for formulas.
+        """
+
+        endog = np.random.normal(size=100)
+        exog1 = np.random.normal(size=100)
+        exog2 = np.random.normal(size=100)
+        exog3 = np.random.normal(size=100)
+        groups = np.kron(lrange(20), np.ones(5))
+
+        endog[0] = np.nan
+        endog[5:7] = np.nan
+        exog2[10:12] = np.nan
+
+        data = pd.DataFrame({"endog": endog, "exog1": exog1, "exog2": exog2,
+                             "exog3": exog3, "groups": groups})
+
+        mod1 = GEE.from_formula("endog ~ exog1 + exog2 + exog3",
+                                groups, data, missing='drop')
+        rslt1 = mod1.fit()
+
+        assert_almost_equal(len(mod1.endog), 95)
+        assert_almost_equal(np.asarray(mod1.exog.shape), np.r_[95, 3])
+
+        data = data.dropna()
+
+        mod2 = GEE.from_formula("endog ~ exog1 + exog2 + exog3",
+                                groups, data, missing='none')
+        rslt2 = mod2.fit()
+
+        assert_almost_equal(rslt1.params, rslt2.params)
+        assert_almost_equal(rslt1.bse, rslt2.bse)
 
     def test_default_time(self):
         """