| Year | Value | | 2023 | 81.611 | | 2022 | 81.134 | | 2021 | 80.419 | | 2020 | 79.421 | | 2019 | 82.176 | | 2018 | 81.955 | | 2017 | 81.705 | | 2016 | 81.419 | | 2015 | 81.093 | | 2014 | 80.719 | | 2013 | 80.292 | | 2012 | 79.81 | | 2011 | 79.288 | | 2010 | 78.742 | | 2009 | 78.189 | | 2008 | 77.647 | | 2007 | 77.13 | | 2006 | 76.65 | | 2005 | 76.21 | | 2004 | 75.812 | | 2003 | 75.46 | | 2002 | 75.156 | | 2001 | 74.899 | | 2000 | 74.678 | | 1999 | 74.487 | | 1998 | 74.317 | | 1997 | 74.16 | | 1996 | 74.01 | | 1995 | 73.857 | | 1994 | 73.694 | | 1993 | 73.513 | | 1992 | 73.306 | | 1991 | 73.066 | | 1990 | 72.794 | | 1989 | 72.492 | | 1988 | 72.163 | | 1987 | 71.811 | | 1986 | 71.436 | | 1985 | 71.041 | | 1984 | 70.629 | | 1983 | 70.204 | | 1982 | 69.769 | | 1981 | 69.327 | | 1980 | 68.913 | | 1979 | 68.457 | | 1978 | 67.987 | | 1977 | 67.497 | | 1976 | 66.99 | | 1975 | 66.457 |
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