| Year | Value | | 2021 | 2.004 | | 2020 | 2.023 | | 2019 | 2.042 | | 2018 | 2.063 | | 2017 | 2.083 | | 2016 | 2.104 | | 2015 | 2.126 | | 2014 | 2.148 | | 2013 | 2.171 | | 2012 | 2.194 | | 2011 | 2.217 | | 2010 | 2.24 | | 2009 | 2.262 | | 2008 | 2.282 | | 2007 | 2.301 | | 2006 | 2.321 | | 2005 | 2.344 | | 2004 | 2.373 | | 2003 | 2.409 | | 2002 | 2.455 | | 2001 | 2.512 | | 2000 | 2.582 | | 1999 | 2.536 | | 1998 | 2.652 | | 1997 | 2.989 | | 1996 | 2.987 | | 1995 | 2.989 | | 1994 | 3.035 | | 1993 | 3.109 | | 1992 | 3.323 | | 1991 | 3.506 | | 1990 | 3.489 | | 1989 | 3.583 | | 1988 | 3.429 | | 1987 | 3.772 | | 1986 | 3.698 | | 1985 | 3.54 | | 1984 | 3.316 | | 1983 | 3.447 | | 1982 | 3.332 | | 1981 | 3.204 | | 1980 | 3.632 | | 1979 | 3.68 | | 1978 | 3.675 | | 1977 | 3.75 | | 1976 | 3.881 | | 1975 | 4.023 | | 1974 | 4.167 | | 1973 | 4.258 | | 1972 | 4.337 | | 1971 | 4.427 | | 1970 | 4.516 | | 1969 | 4.69 | | 1968 | 4.835 | | 1967 | 5.04 | | 1966 | 5.3 | | 1965 | 5.597 | | 1964 | 5.906 | | 1963 | 6.196 | | 1962 | 6.442 | | 1961 | 6.627 | | 1960 | 6.743 |
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