| Year | Value | | 2024 | 15.464 | | 2023 | 15.706 | | 2022 | 16.041 | | 2021 | 16.353 | | 2020 | 16.565 | | 2019 | 16.877 | | 2018 | 17.037 | | 2017 | 17.214 | | 2016 | 17.69 | | 2015 | 18.227 | | 2014 | 18.959 | | 2013 | 19.557 | | 2012 | 19.855 | | 2011 | 20.095 | | 2010 | 20.331 | | 2009 | 20.567 | | 2008 | 20.82 | | 2007 | 21.11 | | 2006 | 21.482 | | 2005 | 21.912 | | 2004 | 22.369 | | 2003 | 22.832 | | 2002 | 23.379 | | 2001 | 23.779 | | 2000 | 24.192 | | 1999 | 24.64 | | 1998 | 25.117 | | 1997 | 25.752 | | 1996 | 26.417 | | 1995 | 27.062 | | 1994 | 27.744 | | 1993 | 28.277 | | 1992 | 28.701 | | 1991 | 29.075 | | 1990 | 29.42 | | 1989 | 29.882 | | 1988 | 30.428 | | 1987 | 31.376 | | 1986 | 32.309 | | 1985 | 32.747 | | 1984 | 33.072 | | 1983 | 33.416 | | 1982 | 33.856 | | 1981 | 34.344 | | 1980 | 35.026 | | 1979 | 36.18 | | 1978 | 37.383 | | 1977 | 38.125 | | 1976 | 39.056 |
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