| Year | Value | | 2021 | 1.3 | | 2020 | 1.4 | | 2019 | 1.3 | | 2018 | 1.3 | | 2017 | 1.4 | | 2016 | 1.4 | | 2015 | 1.4 | | 2014 | 1.4 | | 2013 | 1.6 | | 2012 | 1.6 | | 2011 | 1.6 | | 2010 | 1.7 | | 2009 | 1.76 | | 2008 | 1.77 | | 2007 | 1.76 | | 2006 | 1.76 | | 2005 | 1.76 | | 2004 | 1.78 | | 2003 | 1.8 | | 2002 | 1.77 | | 2001 | 1.75 | | 2000 | 1.74 | | 1999 | 1.807 | | 1998 | 1.799 | | 1997 | 1.692 | | 1996 | 1.698 | | 1995 | 1.679 | | 1994 | 1.684 | | 1993 | 1.691 | | 1992 | 1.708 | | 1991 | 1.729 | | 1990 | 1.687 | | 1989 | 1.734 | | 1988 | 1.772 | | 1987 | 1.715 | | 1986 | 1.697 | | 1985 | 1.726 | | 1984 | 1.618 | | 1983 | 1.751 | | 1982 | 1.575 | | 1981 | 1.54 | | 1980 | 1.613 | | 1979 | 1.628 | | 1978 | 1.541 | | 1977 | 1.651 | | 1976 | 1.834 | | 1975 | 1.842 | | 1974 | 1.988 | | 1973 | 2.116 | | 1972 | 2.211 | | 1971 | 2.299 | | 1970 | 2.358 | | 1969 | 2.432 | | 1968 | 2.525 | | 1967 | 2.645 | | 1966 | 2.826 | | 1965 | 3.074 | | 1964 | 3.232 | | 1963 | 3.468 | | 1962 | 3.443 | | 1961 | 3.483 | | 1960 | 3.51 |
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