| Year | Value | | 2020 | 4.102 | | 2019 | 3.827 | | 2018 | 3.747 | | 2017 | 3.677 | | 2016 | 3.577 | | 2015 | 3.505 | | 2014 | 3.45 | | 2013 | 3.374 | | 2012 | 3.311 | | 2011 | 3.314 | | 2010 | 3.364 | | 2009 | 3.121 | | 2008 | 3.018 | | 2007 | 3.008 | | 2006 | 2.842 | | 2005 | 2.78 | | 2004 | 2.708 | | 2003 | 2.631 | | 2002 | 2.562 | | 2001 | 2.566 | | 2000 | 2.491 | | 1999 | 2.447 | | 1998 | 2.402 | | 1997 | 2.401 | | 1996 | 2.403 | | 1995 | 2.476 | | 1994 | 2.438 | | 1993 | 2.399 | | 1992 | 2.374 | | 1991 | 2.336 | | 1990 | 2.17 | | 1989 | 2.1 | | 1988 | 2.1 | | 1987 | 2.1 | | 1986 | 2 | | 1985 | 1.9 | | 1984 | 1.8 | | 1981 | 1.8 | | 1976 | 1.5 | | 1971 | 1.2 | | 1966 | 1.2 | | 1961 | 1.1 |
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