| Year | Value | | 2021 | 2.464 | | 2020 | 2.433 | | 2019 | 2.435 | | 2018 | 2.428 | | 2017 | 2.6102 | | 2016 | 2.3105 | | 2015 | 2.5388 | | 2014 | 2.4961 | | 2013 | 2.4509 | | 2012 | 2.3962 | | 2010 | 2.0384 | | 2008 | 1.959 | | 2006 | 1.9086 | | 2004 | 1.8971 | | 2002 | 1.8969 | | 2001 | 1.8871 | | 1999 | 2.1 | | 1998 | 1.8611 | | 1997 | 2.1 | | 1996 | 2.1 | | 1995 | 2.1 | | 1994 | 2.1 | | 1993 | 2.2 | | 1992 | 2.1 | | 1991 | 2.1 | | 1990 | 2.1 | | 1989 | 2.1 | | 1988 | 2.1 | | 1987 | 2.1 | | 1986 | 2 | | 1985 | 2 | | 1984 | 1.9 | | 1983 | 1.9 | | 1982 | 1.9 | | 1981 | 1.8 | | 1980 | 1.8 | | 1979 | 1.8 | | 1978 | 1.8 | | 1977 | 1.7 | | 1976 | 1.7 | | 1975 | 1.7 | | 1974 | 1.6 | | 1973 | 1.6 | | 1972 | 1.6 | | 1971 | 1.5 | | 1970 | 1.4 | | 1969 | 1.4 | | 1968 | 1.3 | | 1967 | 1.3 | | 1966 | 1.3 | | 1965 | 1.3 | | 1964 | 1.3 | | 1963 | 1.3 | | 1962 | 1.2 | | 1961 | 1.1 |
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