Request statistics showing global achievements that have been
        unlocked.

        gameID: The id of the game.
        format: Return format. None defaults to json. (json, xml, vdf)

def get_global_achievement_percentages_for_app(self, gameID, format=None):
        """Request statistics showing global achievements that have been
        unlocked.

        gameID: The id of the game.
        format: Return format. None defaults to json. (json, xml, vdf)

        """
        parameters = {'gameid' : gameID}
        if format is not None:
            parameters['format'] = format
        url = self.create_request_url(self.interface,
            'GetGlobalAchievementPercentagesForApp', 2, parameters)
        data = self.retrieve_request(url)
        return self.return_data(data, format=format)