| Year | |
|---|---|
| 1997 | 19.1 |
| 1998 | 21.2 |
| 1999 | 17.6 |
| 2000 | 23.9 |
| 2001 | 23.2 |
| 2002 | 28.7 |
| 2003 | 24.5 |
| 2004 | 24.6 |
| 2005 | 26.5 |
| 2006 | 25.5 |
| 2007 | 26.9 |
| 2008 | 27.9 |
| 2009 | 28.3 |
| 2010 | 31.4 |
| 2011 | 32.1 |
| 2012 | 31.5 |
| 2013 | 34.5 |
| 2014 | 32.0 |
| 2015 | 30.9 |
| 2016 | 26.0 |
| 2017 | 20.8 |
| 2018 | 15.4 |
| 2019 | 12.2 |
| 2020 | 4.3 |
| 2021 | 2.9 |
| 2022 | 2.1 |
| 2023 | 1.3 |
| 2024 | 1.0 |