| Year | |
|---|---|
| 1997 | 38.1 |
| 1998 | 24.9 |
| 1999 | 18.8 |
| 2000 | 19.8 |
| 2001 | 19.5 |
| 2002 | 19.0 |
| 2003 | 18.1 |
| 2004 | 16.6 |
| 2005 | 16.6 |
| 2006 | 17.4 |
| 2007 | 18.4 |
| 2008 | 18.1 |
| 2009 | 18.4 |
| 2010 | 18.8 |
| 2011 | 17.5 |
| 2012 | 18.6 |
| 2013 | 19.3 |
| 2014 | 14.7 |
| 2015 | 11.1 |
| 2016 | 10.5 |
| 2017 | 12.5 |
| 2018 | 10.9 |
| 2019 | 11.4 |
| 2020 | 6.3 |
| 2021 | 7.2 |
| 2022 | 6.9 |
| 2023 | 9.1 |
| 2024 | 9.6 |