| Year | Value | | 2021 | 3.291 | | 2020 | 3.139 | | 2019 | 3.493 | | 2018 | 3.384 | | 2017 | 3.325 | | 2016 | 3.211 | | 2015 | 3.097 | | 2014 | 3.323 | | 2013 | 3.209 | | 2012 | 3.088 | | 2011 | 2.964 | | 2010 | 2.872 | | 2009 | 3.025 | | 2008 | 3.095 | | 2007 | 2.807 | | 2006 | 3.04 | | 2005 | 2.785 | | 2004 | 3.341 | | 2003 | 3.251 | | 2002 | 3.198 | | 2001 | 2.893 | | 2000 | 2.685 | | 1999 | 3.106 | | 1998 | 3.083 | | 1997 | 3.077 | | 1996 | 3.041 | | 1995 | 3.004 | | 1994 | 2.996 | | 1993 | 2.955 | | 1992 | 3.007 | | 1991 | 3.029 | | 1990 | 2.942 | | 1989 | 2.7 | | 1988 | 2.7 | | 1987 | 2.7 | | 1986 | 2.6 | | 1985 | 2.5 | | 1984 | 2.5 | | 1983 | 2.4 | | 1982 | 2.4 | | 1981 | 2.3 | | 1980 | 2.3 | | 1979 | 2.5 | | 1978 | 2.4 | | 1977 | 2.3 | | 1976 | 2.3 | | 1975 | 2.2 | | 1974 | 2.2 | | 1973 | 2.2 | | 1972 | 2.1 | | 1971 | 2.1 | | 1970 | 2 | | 1969 | 2 | | 1968 | 1.9 | | 1967 | 2 | | 1966 | 1.9 | | 1965 | 1.9 | | 1964 | 1.8 | | 1963 | 1.7 | | 1962 | 1.7 | | 1961 | 1.6 | | 1960 | 1.5 |
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