| Year | Value | | 2022 | 4.3 | | 2021 | 5.097 | | 2020 | 4.817 | | 2019 | 4.394 | | 2018 | 4.713 | | 2017 | 4.797 | | 2016 | 4.918 | | 2015 | 4.801 | | 2014 | 4.826 | | 2013 | 4.747 | | 2012 | 4.491 | | 2011 | 4.409 | | 2010 | 4.808 | | 2009 | 4.109 | | 2008 | 3.35 | | 2007 | 3.641 | | 2006 | 3.995 | | 2005 | 4.437 | | 2004 | 4.316 | | 2003 | 4.117 | | 2002 | 2.924 | | 2001 | 2.49 | | 2000 | 2.658 | | 1999 | 3.053 | | 1998 | 3.568 | | 1997 | 4.127 | | 1996 | 3.684 | | 1995 | 3.094 | | 1994 | 3.58 | | 1993 | 3.551 | | 1992 | 2.344 | | 1991 | 1.818 | | 1990 | 2.07 | | 1989 | 0.6 | | 1988 | 0.7 | | 1987 | 0.8 | | 1986 | 0.8 | | 1985 | 1 | | 1984 | 1.1 | | 1983 | 0.9 | | 1982 | 0.4 | | 1981 | 0.2 | | 1980 | 0.2 | | 1979 | 0.4 | | 1978 | 0.4 | | 1977 | 0.4 | | 1976 | 0.7 | | 1975 | 0.3 |
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