| Year | Value | | 2021 | 1.64 | | 2020 | 1.61 | | 2019 | 1.72 | | 2018 | 1.71 | | 2017 | 1.81 | | 2016 | 1.87 | | 2015 | 1.99 | | 2014 | 1.92 | | 2013 | 2.01 | | 2012 | 2.1 | | 2011 | 2.09 | | 2010 | 2.17 | | 2009 | 2.13 | | 2008 | 2.19 | | 2007 | 2.18 | | 2006 | 2.01 | | 2005 | 1.97 | | 2004 | 1.98 | | 2003 | 1.93 | | 2002 | 1.89 | | 2001 | 1.97 | | 2000 | 1.98 | | 1999 | 1.97 | | 1998 | 1.89 | | 1997 | 1.96 | | 1996 | 1.96 | | 1995 | 1.98 | | 1994 | 1.98 | | 1993 | 2.04 | | 1992 | 2.06 | | 1991 | 2.09 | | 1990 | 2.18 | | 1989 | 2.12 | | 1988 | 2.1 | | 1987 | 2.03 | | 1986 | 1.96 | | 1985 | 1.93 | | 1984 | 1.93 | | 1983 | 1.92 | | 1982 | 1.95 | | 1981 | 2.01 | | 1980 | 2.03 | | 1979 | 2.12 | | 1978 | 2.07 | | 1977 | 2.21 | | 1976 | 2.27 | | 1975 | 2.37 | | 1974 | 2.58 | | 1973 | 2.76 | | 1972 | 3 | | 1971 | 3.18 | | 1970 | 3.17 | | 1969 | 3.28 | | 1968 | 3.34 | | 1967 | 3.35 | | 1966 | 3.41 | | 1965 | 3.54 | | 1964 | 3.8 | | 1963 | 4.05 | | 1962 | 4.19 | | 1961 | 4.31 | | 1960 | 4.24 |
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