| Year | Value | | 2023 | 67.235 | | 2022 | 65.665 | | 2021 | 63.09 | | 2020 | 63.33 | | 2019 | 63.337 | | 2018 | 62.311 | | 2017 | 64.295 | | 2016 | 64.403 | | 2015 | 63.985 | | 2014 | 65.996 | | 2013 | 66.028 | | 2012 | 65.553 | | 2011 | 65.443 | | 2010 | 65.308 | | 2009 | 64.999 | | 2008 | 64.724 | | 2007 | 64.332 | | 2006 | 63.911 | | 2005 | 63.466 | | 2004 | 63.017 | | 2003 | 62.519 | | 2002 | 61.934 | | 2001 | 61.461 | | 2000 | 60.895 | | 1999 | 60.382 | | 1998 | 59.863 | | 1997 | 59.386 | | 1996 | 58.93 | | 1995 | 58.592 | | 1994 | 57.667 | | 1993 | 57.691 | | 1992 | 57.319 | | 1991 | 56.842 | | 1990 | 56.744 | | 1989 | 56.206 | | 1988 | 55.64 | | 1987 | 54.994 | | 1986 | 50.528 | | 1985 | 53.569 | | 1984 | 52.712 | | 1983 | 51.699 | | 1982 | 50.46 | | 1981 | 49.688 | | 1980 | 48.569 | | 1979 | 47.479 | | 1978 | 46.385 | | 1977 | 45.357 | | 1976 | 44.351 | | 1975 | 43.345 |
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