| Year | Value | | 2022 | 2.999 | | 2021 | 4.327 | | 2020 | 5.008 | | 2019 | 4.17 | | 2018 | 4.006 | | 2017 | 4.385 | | 2016 | 4.418 | | 2015 | 3.998 | | 2014 | 4.154 | | 2013 | 3.693 | | 2012 | 3.843 | | 2011 | 4.297 | | 2010 | 4.893 | | 2009 | 7.33 | | 2008 | 5.88 | | 2007 | 6.41 | | 2006 | 6.57 | | 2005 | 7.22 | | 2004 | 6.8 | | 2003 | 6.9 | | 2002 | 6.2 | | 2001 | 6.96 | | 2000 | 6.96 | | 1999 | 7.3 | | 1998 | 7.1 | | 1997 | 7.98 | | 1996 | 7.68 | | 1995 | 7.65 | | 1994 | 7.68 | | 1993 | 9.94 | | 1992 | 4.68 | | 1991 | 7.53 | | 1990 | 9.96 | | 1989 | 8.35 | | 1988 | 9.37 | | 1986 | 8.63 | | 1985 | 16.95 | | 1980 | 13.34 | | 1979 | 6.79 | | 1978 | 3.73 |
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