| 1991 | 1992 | 1993 | 1994 | 1995 | 1996 | 1997 | 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 | |
| 14.6 | 14.8 | 12.7 | 10.8 | 10.3 | 8.2 | 6.4 | 5.2 | 4.9 | 5.5 | 5.1 | 4.4 | 4.1 | 4 | 3.9 | 4.2 | 4.5 | 4.7 | 4.3 | 4.3 | 4.4 | 4.5 | 4.5 | 4 | 3.5 | 3.1 | 3.6 | 4 | 4.1 | 3.9 | 3.4 | 3.6 | 4.5 | |
| 13.6 | 13.8 | 11.3 | 9.5 | 9.4 | 7.6 | 5.9 | 4.6 | 4.2 | 4.9 | 4.6 | 4 | 3.7 | 3.6 | 3.4 | 4 | 4.3 | 4.5 | 3.8 | 3.5 | 3.8 | 3.9 | 3.9 | 3.4 | 2.9 | 2.6 | 2.3 | 2.8 | 3.2 | 3.2 | 2.8 | 3.2 | 3.5 |