diff --git a/skgstat/Variogram.py b/skgstat/Variogram.py
index 1858c70..b530ed4 100644
--- a/skgstat/Variogram.py
+++ b/skgstat/Variogram.py
@@ -1425,7 +1425,7 @@ def lag_classes(self):
         # get the groups
         groups = self.lag_groups()
 
-        # yield all groups
+        # yield all groups 
         for i in range(len(self.bins)):
             yield diffs[np.where(groups == i)]
 
@@ -1822,12 +1822,10 @@ def _calc_groups(self, force=False):
         # get the bin edges and distances
         bin_edges = self.bins
         d = self.distance
-
-        # -1 is the group fir distances outside maxlag
-        self._groups = np.ones(len(d), dtype=int) * -1
-
-        for i, bounds in enumerate(zip([0] + list(bin_edges), bin_edges)):
-            self._groups[np.where((d >= bounds[0]) & (d < bounds[1]))] = i
+        
+        # -1 is the group for distances outside maxlag
+        bins = np.digitize(d, bin_edges)
+        self._groups = np.where(bins == self.n_lags, -1, bins) 
 
     def clone(self):
         """Deep copy of self