small fix

mlair/helpers/statistics.py

@@ -333,7 +333,7 @@ class SkillScores:
             skill_score.loc[["CASE II", "AII", "BII"], iahead] = np.stack(self._climatological_skill_score(
                 data, mu_type=2, forecast_name=forecast_name, observation_name=self.observation_name).values.flatten())
 
-            if self.external_data is not None:
+            if self.external_data is not None and self.observation_name in self.external_data.coords["type"]:
                 external_data = self.external_data.sel(ahead=iahead, type=[self.observation_name])
                 skill_score.loc[["CASE III", "AIII"], iahead] = np.stack(self._climatological_skill_score(
                     data, mu_type=3, forecast_name=forecast_name, observation_name=self.observation_name,