| Year | Value | | 2017 | 4.28 | | 2016 | 4.37 | | 2015 | 4.39 | | 2014 | 4.4 | | 2013 | 4.45 | | 2012 | 4.4 | | 2011 | 4.51 | | 2010 | 4.59 | | 2009 | 4.48 | | 2008 | 4.55 | | 2007 | 4.56 | | 2006 | 4.63 | | 2005 | 4.7 | | 2004 | 4.77 | | 2003 | 4.81 | | 2002 | 4.84 | | 2001 | 4.94 | | 2000 | 5.06 | | 1999 | 5.0999999046 | | 1998 | 5.1500000954 | | 1997 | 5.1599998474 | | 1996 | 5.1999998093 | | 1995 | 5.4099998474 | | 1994 | 5.5500001907 | | 1993 | 5.6005997658 | | 1992 | 5.6964001656 | | 1991 | 5.7873001099 | | 1990 | 5.9477000237 | | 1989 | 5.4990000724792 | | 1988 | 5.4742999076843 | | 1987 | 5.4335999488831 | | 1986 | 5.499499797821 | | 1985 | 5.5118999481201 | | 1984 | 5.4029002189636 | | 1983 | 5.3383002281189 | | 1982 | 5.3646001815796 | | 1981 | 5.1816000938416 | | 1980 | 5.1985001564026 | | 1979 | 5.2437000274658 | | 1978 | 5.1737999916077 | | 1977 | 5.2698001861572 | | 1976 | 5.3109002113342 | | 1975 | 5.4443998336792 | | 1974 | 5.4643998146057 | | 1973 | 5.4028000831604 | | 1972 | 5.4942998886108 | | 1971 | 5.4433999061584 | | 1970 | 5.4503002166748 |
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