def _get_contrast(self, con_id):
        """
        Retreive contrast with identifier 'con_id' from the list of contrast
        objects stored in self.contrasts
        """
        for contrasts in list(self.contrasts.values()):
            for contrast in contrasts:
                if contrast.estimation.id == con_id:
                    return contrast
        raise Exception("Contrast activity with id: " + str(con_id) +
                        " not found.")