| Year | |
|---|---|
| 1997 | 15.4 |
| 1998 | 11.2 |
| 1999 | 11.3 |
| 2000 | 18.5 |
| 2001 | 23.6 |
| 2002 | 30.4 |
| 2003 | 20.6 |
| 2004 | 25.5 |
| 2005 | 38.3 |
| 2006 | 23.6 |
| 2007 | 24.0 |
| 2008 | 19.3 |
| 2009 | 7.5 |
| 2010 | 11.2 |
| 2011 | 19.4 |
| 2012 | 20.2 |
| 2013 | 19.1 |
| 2014 | 20.4 |
| 2015 | 15.9 |
| 2016 | 16.5 |
| 2017 | 13.6 |
| 2018 | 13.3 |
| 2019 | 15.9 |
| 2020 | 15.9 |
| 2021 | 13.4 |
| 2022 | 14.8 |
| 2023 | 13.6 |
| 2024 | 15.7 |