| Year | Value | | 2023 | 79.51 | | 2022 | 79.06 | | 2021 | 79.19 | | 2020 | 79.34 | | 2019 | 80.11 | | 2018 | 79.76 | | 2017 | 79.73 | | 2016 | 79.94 | | 2015 | 79.86 | | 2014 | 79.76 | | 2013 | 79.74 | | 2012 | 79.61 | | 2011 | 79.39 | | 2010 | 79.21 | | 2009 | 78.91 | | 2008 | 78.51 | | 2007 | 78.29 | | 2006 | 78.29 | | 2005 | 77.82 | | 2004 | 77.68 | | 2003 | 77.32 | | 2002 | 77.16 | | 2001 | 76.92 | | 2000 | 76.64 | | 1999 | 76.2 | | 1998 | 75.97 | | 1997 | 75.71 | | 1996 | 75.43 | | 1995 | 75.07 | | 1994 | 74.94 | | 1993 | 74.7 | | 1992 | 74.76 | | 1991 | 74.53 | | 1990 | 74.3 | | 1989 | 73.92 | | 1988 | 73.61 | | 1987 | 73.53 | | 1986 | 73.22 | | 1985 | 73.02 | | 1984 | 73.01 | | 1983 | 72.68 | | 1982 | 72.33 | | 1981 | 72.01 | | 1980 | 71.59 | | 1979 | 71.32 | | 1978 | 70.92 | | 1977 | 70.64 | | 1976 | 70.3 |
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