| Year | Value | | 2018 | 2.98 | | 2017 | 3.02 | | 2016 | 2.99 | | 2015 | 3.03 | | 2014 | 3.08 | | 2013 | 3.09 | | 2012 | 3.1 | | 2011 | 3.14 | | 2010 | 3.16 | | 2009 | 3.19 | | 2008 | 3.3 | | 2007 | 3.35 | | 2006 | 3.4 | | 2005 | 3.73 | | 2004 | 3.8 | | 2003 | 3.86 | | 2002 | 3.81 | | 2001 | 3.86 | | 2000 | 3.85 | | 1999 | 6.1199998856 | | 1998 | 6.0999999046 | | 1997 | 6.0500001907 | | 1996 | 6.0300002098 | | 1995 | 5.9800000191 | | 1994 | 6.1012001038 | | 1993 | 6.0778999329 | | 1992 | 5.9895000458 | | 1991 | 5.9699997902 | | 1990 | 6.2431998253 | | 1989 | 6.2836999893189 | | 1988 | 6.3168997764587 | | 1987 | 6.2898998260498 | | 1986 | 6.3801999092102 | | 1985 | 6.5095000267029 | | 1984 | 6.496600151062 | | 1983 | 6.5434999465942 | | 1982 | 6.7666001319885 | | 1981 | 6.7294001579285 | | 1980 | 6.7973999977112 | | 1975 | 8.0792999267578 | | 1960 | 6.7379374504089 |
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