| Year | Value | | 2022 | 78.1 | | 2021 | 75.1 | | 2020 | 77.5 | | 2019 | 78.8 | | 2018 | 78.6 | | 2017 | 78.4 | | 2016 | 78.5 | | 2015 | 78.2 | | 2014 | 78 | | 2013 | 78.6 | | 2012 | 77.9 | | 2011 | 77.8 | | 2010 | 77.2 | | 2009 | 77.1 | | 2008 | 76.6 | | 2007 | 76.3 | | 2006 | 76.3 | | 2005 | 76.3 | | 2004 | 76.2 | | 2003 | 75.6 | | 2002 | 75.4 | | 2001 | 75.2 | | 2000 | 75.3 | | 1999 | 75.1 | | 1998 | 74.8 | | 1997 | 73.87 | | 1996 | 74.58 | | 1995 | 74.9 | | 1994 | 75.25 | | 1993 | 75.26 | | 1992 | 75.09 | | 1991 | 74.88 | | 1990 | 75.15 | | 1989 | 75.19 | | 1988 | 74.99 | | 1987 | 74.81 | | 1986 | 75.06 | | 1985 | 74.46 | | 1984 | 74.66 | | 1983 | 74.48 | | 1982 | 74.08 | | 1981 | 74.43 | | 1980 | 73.99 | | 1979 | 74.11 | | 1978 | 73.72 | | 1977 | 73.5 | | 1976 | 73.92 | | 1975 | 73.58 | | 1974 | 73.6 |
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