| Year | Value | | 2020 | 2.614 | | 2018 | 2.498 | | 2016 | 2.426 | | 2014 | 2.354 | | 2012 | 2.284 | | 2010 | 2.213 | | 2008 | 2.147 | | 2006 | 2.083 | | 2004 | 2.031 | | 2002 | 1.979 | | 2000 | 1.935 | | 1998 | 1.89 | | 1996 | 1.841 | | 1994 | 1.772 | | 1992 | 1.702 | | 1990 | 1.649 | | 1985 | 1.5 | | 1981 | 1.3 | | 1979 | 1.2 | | 1978 | 1.2 | | 1977 | 1.2 | | 1976 | 1.1 | | 1975 | 1.1 | | 1974 | 1.1 | | 1973 | 1.1 | | 1972 | 1.1 | | 1971 | 1.1 | | 1970 | 1.1 | | 1969 | 1.1 | | 1968 | 1 | | 1967 | 1 | | 1966 | 1 | | 1965 | 1 | | 1964 | 1 | | 1963 | 1 | | 1962 | 1 | | 1961 | 1 | | 1960 | 1 |
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