| Year | Value | | 2023 | 68.51 | | 2022 | 68.089 | | 2021 | 66.403 | | 2020 | 66.847 | | 2019 | 66.863 | | 2018 | 66.592 | | 2017 | 66.172 | | 2016 | 65.794 | | 2015 | 65.313 | | 2014 | 64.857 | | 2013 | 64.454 | | 2012 | 64.053 | | 2011 | 63.635 | | 2010 | 63.05 | | 2009 | 62.67 | | 2008 | 62.254 | | 2007 | 61.767 | | 2006 | 61.339 | | 2005 | 60.888 | | 2004 | 60.3 | | 2003 | 60.062 | | 2002 | 59.669 | | 2001 | 59.167 | | 2000 | 58.263 | | 1999 | 58.195 | | 1998 | 58.036 | | 1997 | 57.863 | | 1996 | 58.031 | | 1995 | 58.009 | | 1994 | 57.501 | | 1993 | 57.652 | | 1992 | 57.746 | | 1991 | 57.702 | | 1990 | 57.445 | | 1989 | 57.285 | | 1988 | 57.056 | | 1987 | 56.739 | | 1986 | 56.408 | | 1985 | 56.004 | | 1984 | 55.714 | | 1983 | 55.307 | | 1982 | 54.839 | | 1981 | 54.296 | | 1980 | 53.983 | | 1979 | 53.559 | | 1978 | 53.113 | | 1977 | 52.697 | | 1976 | 52.264 | | 1975 | 51.846 |
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