| 年份 | 样本(家) | 均值 | 中位数 |
| 2012 | 179 | 9.92% | 10.38% |
| 2013 | 179 | 8.10% | 8.43% |
| 2014 | 179 | 6.97% | 7.20% |
| 2015 | 179 | 9.22% | 9.61% |
| 2016 | 180 | 11.39% | 11.95% |
| 2017 | 179 | 7.58% | 7.58% |
| 2018 | 178 | 8.87% | 9.18% |