| Year | Value | | 2024 | 11 | | 2023 | 11.2 | | 2022 | 12.2 | | 2021 | 12.9 | | 2020 | 13.4 | | 2019 | 14.4 | | 2018 | 15.4 | | 2017 | 16.2 | | 2016 | 16.6 | | 2015 | 17.1 | | 2014 | 17.5 | | 2013 | 17 | | 2012 | 17.2 | | 2011 | 16.9 | | 2010 | 17.2 | | 2009 | 17.6 | | 2008 | 18.2 | | 2007 | 18.4 | | 2006 | 18.686 | | 2005 | 18.794 | | 2004 | 18.857 | | 2003 | 18.949 | | 2002 | 19.531 | | 2001 | 20.593 | | 2000 | 21.42 | | 1999 | 22.646 | | 1998 | 23.264 | | 1997 | 23.543 | | 1996 | 23.743 | | 1995 | 24.108 | | 1994 | 24.267 | | 1993 | 24.443 | | 1992 | 24.439 | | 1991 | 24.89 | | 1990 | 25.453 | | 1989 | 26.938 | | 1988 | 27.673 | | 1987 | 28.287 | | 1986 | 29.036 | | 1985 | 29.689 | | 1984 | 30.425 | | 1983 | 31.231 | | 1982 | 32.045 | | 1981 | 33.101 | | 1980 | 33.671 | | 1979 | 34.19 | | 1978 | 34.487 | | 1977 | 34.693 | | 1976 | 35.319 |
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