| 2006 | 4.7615 |
| 2007 | 4.9071 |
| 2008 | 5.2982 |
| 2009 | 5.5896 |
| 2010 | 5.6606 |
| 2011 | 5.5631 |
| 2012 | 5.5155 |
| 2013 | 5.5961 |
| 2014 | 5.4963 |
| 2015 | 5.5895 |
| 2016 | 5.7992 |
| 2017 | 6.0201 |
| 2018 | 5.9871 |
| 2019 | 6.1098 |
| 2020 | 6.2399 |
| 2021 | 6.3500 |