diff --git a/mlair/plotting/postprocessing_plotting.py b/mlair/plotting/postprocessing_plotting.py
index 2c8fd6e4935e063a8026ebdc6cb1309d4d8f11fa..a4bd68fcfb66692cb6eb6ba7f1801ae65068e25b 100644
--- a/mlair/plotting/postprocessing_plotting.py
+++ b/mlair/plotting/postprocessing_plotting.py
@@ -165,10 +165,10 @@ class PlotContingency(AbstractPlotClass):
             for threshold in range(self._min_threshold, self._max_threshold):
                 for pred in predictions:
                     ta, fa, fb, tb = self._single_contingency(obs, pred, threshold)
-                    contingency_array.loc[dict(thresholds=threshold, contingency_cell="ta", type=pred.type.values)] = ta + 1
-                    contingency_array.loc[dict(thresholds=threshold, contingency_cell="fa", type=pred.type.values)] = fa + 1
-                    contingency_array.loc[dict(thresholds=threshold, contingency_cell="fb", type=pred.type.values)] = fb + 1
-                    contingency_array.loc[dict(thresholds=threshold, contingency_cell="tb", type=pred.type.values)] = tb + 1
+                    contingency_array.loc[dict(thresholds=threshold, contingency_cell="ta", type=pred.type.values)] = contingency_array.loc[dict(thresholds=threshold, contingency_cell="ta", type=pred.type.values)] + ta
+                    contingency_array.loc[dict(thresholds=threshold, contingency_cell="fa", type=pred.type.values)] = contingency_array.loc[dict(thresholds=threshold, contingency_cell="fa", type=pred.type.values)] + fa
+                    contingency_array.loc[dict(thresholds=threshold, contingency_cell="fb", type=pred.type.values)] = contingency_array.loc[dict(thresholds=threshold, contingency_cell="fb", type=pred.type.values)] + fb
+                    contingency_array.loc[dict(thresholds=threshold, contingency_cell="tb", type=pred.type.values)] = contingency_array.loc[dict(thresholds=threshold, contingency_cell="tb", type=pred.type.values)] + tb
         return contingency_array
 
     def _single_contingency(self, obs, pred, threshold):