| Year | Mean (×10−3) | Standard deviation (×10−4) | Minimum (×10−3) | Maximum (×10−3) |
| 2003 | 5.655 | 3.006 | 5.243 | 6.109 |
| 2004 | 5.602 | 2.367 | 5.289 | 5.944 |
| 2005 | 5.479 | 1.948 | 5.091 | 5.732 |
| 2006 | 5.630 | 3.389 | 5.098 | 6.012 |
| 2007 | 5.566 | 2.004 | 5.251 | 5.792 |
| 2008 | 5.741 | 3.580 | 5.228 | 6.174 |
| 2009 | 5.784 | 2.742 | 5.360 | 6.132 |
| 2010 | 5.746 | 2.934 | 5.316 | 6.138 |
| 2011 | 5.789 | 2.224 | 5.434 | 6.078 |
| 2012 | 5.689 | 3.037 | 5.238 | 6.073 |