| Year | Value | | 2022 | 3.233 | | 2021 | 4.369 | | 2020 | 4.422 | | 2019 | 3.687 | | 2018 | 3.8 | | 2017 | 4.162 | | 2016 | 4.679 | | 2015 | 4.296 | | 2014 | 3.484 | | 2013 | 3.423 | | 2012 | 3.123 | | 2011 | 3.215 | | 2010 | 3.521 | | 2009 | 3.103 | | 2008 | 2.55 | | 2007 | 2.493 | | 2006 | 3.399 | | 2005 | 4.381 | | 2004 | 4.256 | | 2003 | 4.221 | | 2002 | 4.022 | | 2001 | 3.738 | | 2000 | 3.458 | | 1999 | 3.247 | | 1998 | 3.737 | | 1997 | 4.687 | | 1996 | 5.036 | | 1995 | 6.307 | | 1994 | 5.349 | | 1993 | 5.965 | | 1992 | 5.91 | | 1991 | 5.407 | | 1990 | 5.261 | | 1989 | 4.83 | | 1988 | 3.02 | | 1987 | 2.03 | | 1986 | 1.79 | | 1985 | 2.32 | | 1984 | 2.9 | | 1983 | 2.04 | | 1982 | 1.7 | | 1981 | 2.03 | | 1980 | 1.65 | | 1979 | 1.99 | | 1978 | 1.8 | | 1977 | 1.46 | | 1976 | 1.76 | | 1975 | 2.29 | | 1974 | 1.48 | | 1973 | 1.55 | | 1972 | 1.67 |
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