| Year | Value | | 2023 | 4.556 | | 2022 | 4.634 | | 2021 | 4.707 | | 2020 | 4.795 | | 2019 | 4.9 | | 2018 | 5.031 | | 2017 | 5.155 | | 2016 | 5.254 | | 2015 | 5.32 | | 2014 | 5.374 | | 2013 | 5.41 | | 2012 | 5.429 | | 2011 | 5.436 | | 2010 | 5.452 | | 2009 | 5.491 | | 2008 | 5.548 | | 2007 | 5.627 | | 2006 | 5.673 | | 2005 | 5.692 | | 2004 | 5.718 | | 2003 | 5.763 | | 2002 | 5.793 | | 2001 | 5.848 | | 2000 | 5.937 | | 1999 | 6.032 | | 1998 | 6.136 | | 1997 | 6.23 | | 1996 | 6.279 | | 1995 | 6.33 | | 1994 | 6.419 | | 1993 | 6.51 | | 1992 | 6.593 | | 1991 | 6.666 | | 1990 | 6.741 | | 1989 | 6.815 | | 1988 | 6.871 | | 1987 | 6.918 | | 1986 | 6.962 | | 1985 | 7.003 | | 1984 | 7.031 | | 1983 | 7.045 | | 1982 | 7.055 | | 1981 | 7.056 | | 1980 | 6.989 | | 1979 | 6.905 | | 1978 | 6.824 | | 1977 | 6.832 | | 1976 | 6.83 | | 1975 | 6.83 |
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