| Year | Value | | 2018 | 4.168 | | 2015 | 3.973 | | 2014 | 3.907 | | 2013 | 3.888 | | 2012 | 3.819 | | 2011 | 3.763 | | 2010 | 3.683 | | 2009 | 3.661 | | 2008 | 3.577 | | 2007 | 3.612 | | 2006 | 3.617 | | 2005 | 3.605 | | 2004 | 3.484 | | 2003 | 3.553 | | 2002 | 3.472 | | 2001 | 3.384 | | 2000 | 3.399 | | 1998 | 3.458 | | 1997 | 3.445 | | 1996 | 3.522 | | 1995 | 3.429 | | 1994 | 3.286 | | 1993 | 3.305 | | 1992 | 3.119 | | 1991 | 3.068 | | 1990 | 3.25 | | 1987 | 3.022 | | 1986 | 2.953 | | 1985 | 2.865 | | 1984 | 2.759 | | 1983 | 2.688 | | 1982 | 2.588 | | 1981 | 2.484 | | 1980 | 2.46 | | 1979 | 2.408 | | 1978 | 2.321 | | 1977 | 2.258 | | 1976 | 2.205 | | 1975 | 2.152 | | 1974 | 2.102 | | 1973 | 2.042 | | 1972 | 1.985 | | 1971 | 1.896 | | 1970 | 1.863 | | 1965 | 1.667 | | 1960 | 1.405 |
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