| Year | Mean (×10−5 ) | Standard deviation (×10−6) | Minimum (×10−5) | Maximum (×10−5) |
| 2003 | 5.246 | 3.141 | 4.9 | 6.0 |
| 2004 | 4.637 | 4.542 | 4.0 | 5.3 |
| 2005 | 4.307 | 4.860 | 3.9 | 5.7 |
| 2006 | 4.092 | 3.555 | 3.6 | 4.6 |
| 2007 | 3.771 | 2.708 | 3.5 | 4.4 |
| 2008 | 3.834 | 2.110 | 3.5 | 4.1 |
| 2009 | 3.752 | 1.554 | 3.6 | 4.0 |
| 2010 | 3.849 | 1.969 | 3.4 | 4.2 |
| 2011 | 3.943 | 1.611 | 3.7 | 4.2 |
| 2012 | 3.982 | 2.056 | 3.7 | 4.3 |