| Year | Value | | 2021 | 3.32 | | 2020 | 3.13 | | 2019 | 2.9 | | 2018 | 2.84 | | 2017 | 2.75 | | 2016 | 2.77 | | 2015 | 2.74 | | 2014 | 2.73 | | 2013 | 2.64 | | 2012 | 2.62 | | 2011 | 2.59 | | 2010 | 2.59 | | 2009 | 2.55 | | 2008 | 2.68 | | 2007 | 2.47 | | 2006 | 2.36 | | 2005 | 2.22 | | 2004 | 2.21 | | 2003 | 2.03 | | 2002 | 2 | | 2001 | 1.9 | | 2000 | 1.8 | | 1999 | 1.8 | | 1998 | 1.8 | | 1997 | 2 | | 1996 | 2.13 | | 1995 | 2.26 | | 1994 | 2.28 | | 1993 | 2.3 | | 1992 | 2.5 | | 1991 | 2.61 | | 1990 | 2.72 | | 1989 | 2.82 | | 1988 | 3.13 | | 1987 | 3.19 | | 1986 | 3.135 | | 1985 | 3.08 | | 1984 | 3.08 | | 1983 | 3.04 | | 1982 | 3 | | 1981 | 2.95 | | 1980 | 2.9 | | 1979 | 3.019 | | 1978 | 3.165 | | 1977 | 3.268 | | 1976 | 3.333 | | 1975 | 3.393 | | 1974 | 3.453 | | 1973 | 3.51 | | 1972 | 3.566 | | 1971 | 3.545 | | 1970 | 3.529 | | 1969 | 3.537 | | 1968 | 3.681 | | 1967 | 3.877 | | 1966 | 4.008 | | 1965 | 4.111 | | 1964 | 4.219 | | 1963 | 4.341 | | 1962 | 4.404 | | 1961 | 4.486 | | 1960 | 4.53 |
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