| 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 | |
| Increasing | 27.0 | 17.3 | 19.6 | 29.0 | 30.3 | 28.2 | 30.5 | 33.2 | 28.6 | 30.5 | 25.3 | 22.0 | 21.2 | 20.7 | 21.7 | 23.9 | 21.3 | 13.3 | 10.3 | 11.7 | 15.0 | 7.8 | 14.1 | 14.4 | 20.1 | 19.6 | 25.4 | 21.6 | 17.4 | 17.6 | 20.0 | 19.7 | 20.0 |
| Decreasing | 20.7 | 30.5 | 29.2 | 17.7 | 14.0 | 15.9 | 10.3 | 9.5 | 10.7 | 12.0 | 12.7 | 17.8 | 13.2 | 13.5 | 15.7 | 16.7 | 20.2 | 36.1 | 43.8 | 40.8 | 36.4 | 59.6 | 27.4 | 26.7 | 15.1 | 13.4 | 9.5 | 9.9 | 10.7 | 24.8 | 11.0 | 11.2 | 13.2 |
| Constant | 52.3 | 52.2 | 51.2 | 53.3 | 55.7 | 55.9 | 59.2 | 57.3 | 60.7 | 57.5 | 62.0 | 60.2 | 65.6 | 65.8 | 62.6 | 59.4 | 58.5 | 50.6 | 45.8 | 47.5 | 48.6 | 39.5 | 58.5 | 58.8 | 64.8 | 67.1 | 65.1 | 68.5 | 71.9 | 57.6 | 69.0 | 69.1 | 66.8 |