| Year | Value | | 2023 | 5.398 | | 2022 | 5.881 | | 2021 | 7.869 | | 2020 | 6.621 | | 2019 | 5.38 | | 2018 | 5.35 | | 2017 | 5.315 | | 2016 | 5.31 | | 2015 | 5.252 | | 2014 | 5.145 | | 2013 | 5.033 | | 2012 | 4.977 | | 2011 | 5.019 | | 2010 | 5.012 | | 2009 | 5.029 | | 2008 | 5.013 | | 2007 | 5.065 | | 2006 | 5.042 | | 2005 | 5.085 | | 2004 | 5.151 | | 2003 | 5.242 | | 2002 | 5.46 | | 2001 | 5.531 | | 2000 | 5.574 | | 1999 | 5.5 | | 1998 | 5.432 | | 1997 | 5.482 | | 1996 | 5.586 | | 1995 | 5.697 | | 1994 | 5.805 | | 1993 | 5.892 | | 1992 | 5.96 | | 1991 | 5.9 | | 1990 | 5.863 | | 1989 | 5.833 | | 1988 | 5.888 | | 1987 | 5.884 | | 1986 | 5.78 | | 1985 | 6.893 | | 1984 | 5.715 | | 1983 | 5.763 | | 1982 | 5.824 | | 1981 | 6.019 | | 1980 | 6.159 | | 1979 | 6.323 | | 1978 | 6.423 | | 1977 | 6.636 | | 1976 | 6.85 | | 1975 | 7.089 |
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