| Year | Value | | 2023 | 80.2 | | 2022 | 78.8 | | 2021 | 77 | | 2020 | 78.8 | | 2019 | 79.5 | | 2018 | 80.6 | | 2017 | 79.4 | | 2016 | 79 | | 2015 | 78.6 | | 2014 | 78.9 | | 2013 | 79 | | 2012 | 78.4 | | 2011 | 78.9 | | 2010 | 78.5 | | 2009 | 77.5 | | 2008 | 77.8 | | 2007 | 76.9 | | 2006 | 76.4 | | 2005 | 76.8 | | 2004 | 77.246 | | 2003 | 77.133 | | 2002 | 77.127 | | 2001 | 76.722 | | 2000 | 76.597 | | 1999 | 76.446 | | 1998 | 76.594 | | 1997 | 76.557 | | 1996 | 76.585 | | 1995 | 76.613 | | 1994 | 76.768 | | 1993 | 77.038 | | 1992 | 76.954 | | 1991 | 77.075 | | 1990 | 77.147 | | 1989 | 76.708 | | 1988 | 76.106 | | 1987 | 75.867 | | 1986 | 75.614 | | 1985 | 75.174 | | 1984 | 75.078 | | 1983 | 74.951 | | 1982 | 74.769 | | 1981 | 74.511 | | 1980 | 74.528 | | 1979 | 73.655 | | 1978 | 74.158 | | 1977 | 74.067 | | 1976 | 74.106 | | 1975 | 73.897 |
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