| Year | Value | | 2022 | 3.701 | | 2021 | 5.116 | | 2020 | 6.456 | | 2019 | 5.159 | | 2018 | 5.3 | | 2017 | 5.592 | | 2016 | 5.711 | | 2015 | 6.055 | | 2014 | 6.078 | | 2013 | 5.663 | | 2012 | 5.225 | | 2011 | 5.083 | | 2010 | 5.214 | | 2009 | 5.565 | | 2008 | 4.242 | | 2007 | 4.381 | | 2006 | 4.785 | | 2005 | 5.036 | | 2004 | 5.399 | | 2003 | 5.933 | | 2002 | 6.375 | | 2001 | 6.747 | | 2000 | 6.288 | | 1999 | 6.876 | | 1998 | 7.684 | | 1997 | 8.367 | | 1996 | 8.509 | | 1995 | 8.473 | | 1994 | 9.724 | | 1993 | 10.879 | | 1992 | 10.733 | | 1991 | 9.586 | | 1990 | 6.926 | | 1989 | 6.175 | | 1988 | 7.226 | | 1987 | 8.106 | | 1986 | 8.081 | | 1985 | 8.258 | | 1984 | 8.987 | | 1983 | 9.961 | | 1982 | 7.163 | | 1981 | 5.784 | | 1980 | 6.105 | | 1979 | 6.265 |
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