| Year | Value | | 2016 | 3.84 | | 2015 | 3.82 | | 2014 | 3.79 | | 2013 | 3.74 | | 2012 | 3.75 | | 2011 | 3.79 | | 2010 | 3.78 | | 2009 | 3.77 | | 2008 | 3.82 | | 2006 | 3.93 | | 2005 | 3.9 | | 2004 | 4 | | 2003 | 3.97 | | 2002 | 3.93 | | 2001 | 3.95 | | 2000 | 4.04 | | 1999 | 7.9000000954 | | 1998 | 8.1999998093 | | 1997 | 8.3000001907 | | 1996 | 8.5 | | 1995 | 8.6999998093 | | 1994 | 8.6999998093 | | 1993 | 8.8999996185 | | 1992 | 8.8999996185 | | 1991 | 9.1999998093 | | 1989 | 9.8000001907349 | | 1987 | 10.5 | | 1985 | 10.89999961853 | | 1984 | 11.699999809265 | | 1983 | 11.89999961853 | | 1982 | 12.10000038147 | | 1981 | 12.199999809265 | | 1980 | 12.300000190735 | | 1979 | 12.199999809265 | | 1978 | 12.10000038147 | | 1977 | 12.10000038147 | | 1976 | 11.89999961853 | | 1975 | 11.89999961853 | | 1974 | 12 | | 1973 | 11.800000190735 | | 1972 | 11.800000190735 | | 1971 | 11.60000038147 | | 1970 | 11.699999809265 | | 1969 | 11.699999809265 | | 1968 | 11.800000190735 | | 1967 | 11.800000190735 | | 1966 | 11.800000190735 | | 1965 | 11.89999961853 | | 1964 | 11.800000190735 | | 1963 | 11.699999809265 | | 1962 | 11.39999961853 | | 1961 | 11.300000190735 | | 1960 | 11.199999809265 |
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