| Year | Value | | 2020 | 5.615 | | 2019 | 5.45 | | 2018 | 5.294 | | 2017 | 5.115 | | 2016 | 4.936 | | 2015 | 4.75 | | 2014 | 4.561 | | 2013 | 4.397 | | 2012 | 4.238 | | 2011 | 4.12 | | 2010 | 3.979 | | 2009 | 3.853 | | 2008 | 3.746 | | 2007 | 3.651 | | 2006 | 3.565 | | 2005 | 3.495 | | 2004 | 3.411 | | 2003 | 3.336 | | 2002 | 3.285 | | 2001 | 3.252 | | 2000 | 3.202 | | 1999 | 3.121 | | 1998 | 3.099 | | 1997 | 3.043 | | 1996 | 3.002 | | 1995 | 2.923 | | 1994 | 2.89 | | 1993 | 2.871 | | 1992 | 2.859 | | 1991 | 2.838 | | 1990 | 2.8 | | 1979 | 1.9 | | 1978 | 1.7 | | 1977 | 1.5 | | 1976 | 1.3 | | 1975 | 1.2 | | 1974 | 1.2 | | 1973 | 1.1 | | 1972 | 1 | | 1971 | 1 | | 1970 | 0.9 | | 1969 | 0.9 | | 1968 | 1 | | 1967 | 0.9 | | 1966 | 0.9 | | 1965 | 0.9 | | 1964 | 0.8 | | 1963 | 0.8 | | 1962 | 0.9 | | 1961 | 0.8 | | 1960 | 0.8 |
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