| Year | Value | | 2017 | 5.54 | | 2016 | 5.49 | | 2015 | 5.56 | | 2014 | 5.91 | | 2013 | 5.86 | | 2012 | 5.89 | | 2011 | 5.84 | | 2010 | 5.78 | | 2009 | 5.53 | | 2008 | 5.63 | | 2007 | 5.65 | | 2006 | 5.62 | | 2005 | 5.45 | | 2004 | 5.53 | | 2003 | 5.61 | | 2002 | 5.67 | | 2001 | 6 | | 2000 | 6.15 | | 1999 | 5.9299998283 | | 1998 | 6.0599999428 | | 1997 | 6.0100002289 | | 1996 | 6.1900000572 | | 1995 | 5.75 | | 1994 | 5.9095001221 | | 1993 | 6.071199894 | | 1992 | 6.210100174 | | 1991 | 6.5201001167 | | 1990 | 7.3777999878 | | 1989 | 7.393000125885 | | 1988 | 7.4416999816894 | | 1987 | 7.4225001335144 | | 1986 | 7.402599811554 | | 1985 | 7.4542999267578 | | 1984 | 7.4465999603272 | | 1983 | 7.4651999473572 | | 1982 | 7.4235000610352 | | 1981 | 7.3081002235413 | | 1980 | 7.234799861908 |
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