| Year | Value | | 2021 | 1.82 | | 2020 | 1.72 | | 2019 | 1.74 | | 2018 | 1.71 | | 2017 | 1.71 | | 2016 | 1.74 | | 2015 | 1.8 | | 2014 | 1.93 | | 2013 | 1.93 | | 2012 | 2.04 | | 2011 | 2.02 | | 2010 | 2.2 | | 2009 | 2.23 | | 2008 | 2.15 | | 2007 | 2.09 | | 2006 | 2.08 | | 2005 | 2.05 | | 2004 | 2.04 | | 2003 | 1.99 | | 2002 | 1.93 | | 2001 | 1.95 | | 2000 | 2.08 | | 1999 | 1.99 | | 1998 | 2.05 | | 1997 | 2.04 | | 1996 | 2.12 | | 1995 | 2.08 | | 1994 | 2.14 | | 1993 | 2.22 | | 1992 | 2.21 | | 1991 | 2.18 | | 1990 | 2.3 | | 1989 | 2.19 | | 1988 | 2.26 | | 1987 | 2.06 | | 1986 | 1.92 | | 1985 | 1.93 | | 1984 | 2.08 | | 1983 | 2.24 | | 1982 | 2.27 | | 1981 | 2.33 | | 1980 | 2.48 | | 1979 | 2.49 | | 1978 | 2.35 | | 1977 | 2.31 | | 1976 | 2.52 | | 1975 | 2.65 | | 1974 | 2.66 | | 1973 | 2.94 | | 1972 | 3.09 | | 1971 | 2.92 | | 1970 | 2.81 | | 1969 | 2.99 | | 1968 | 3.06 | | 1967 | 3.28 | | 1966 | 3.59 | | 1965 | 3.73 | | 1964 | 3.88 | | 1963 | 3.99 | | 1962 | 3.98 | | 1961 | 3.88 | | 1960 | 4.29 |
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