| 1998 | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 | 2011 | 2012 | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | |
| 6.8 | 7 | 6.9 | 7.1 | 6.7 | 6.1 | 6.1 | 6.6 | 6.9 | 6.3 | 6.8 | 6.6 | 7.3 | 8.2 | 8.3 | 8.8 | 9.3 | 8.8 | 8.9 | 8.2 | 7.8 | 6.5 | 6.3 | 7.7 | 8 | 8.5 | |
| 17.3 | 17.9 | 17.9 | 17.3 | 17.6 | 17.1 | 17.1 | 18.4 | 18.1 | 18.7 | 17.3 | 17.3 | 18.9 | 18.8 | 19.1 | 18.4 | 19.1 | 18.1 | 16.9 | 15.3 | 14.3 | 13.9 | 12.3 | 14.4 | 14.9 | 15 |