code: public GroupStatListResult getGroupStatistic(String start, int duration)
              throws APIConnectionException, APIRequestException {
          Preconditions.checkArgument(verifyDateFormat("DAY", start), "Illegal date format");
          Preconditions.checkArgument(0 <= duration && duration <= 60, " 0 <= duration <= 60");
          String url = mBaseReportPath + mV2StatisticPath + "/groups?time_unit=DAY&start=" + start + "&duration=" + duration;
          ResponseWrapper responseWrapper = _httpClient.sendGet(url);
          return GroupStatListResult.fromResponse(responseWrapper);
      }
docs: Get group statistic, time unit only supports DAY now.
  @param start Format: yyyy-MM-dd
  @param duration duration must between 0 and 60
  @return {@link GroupStatListResult}
  @throws APIConnectionException connect exception
  @throws APIRequestException request exception