| Year | Value | | 2022 | 2.929 | | 2021 | 3.403 | | 2020 | 4.357 | | 2019 | 3.627 | | 2018 | 3.665 | | 2017 | 4.008 | | 2016 | 4.694 | | 2015 | 5.389 | | 2014 | 5.737 | | 2013 | 6.115 | | 2012 | 6.203 | | 2011 | 6.388 | | 2010 | 6.861 | | 2009 | 6.896 | | 2008 | 5.977 | | 2007 | 6.472 | | 2006 | 6.798 | | 2005 | 6.923 | | 2004 | 7.317 | | 2003 | 7.488 | | 2002 | 6.902 | | 2001 | 7.107 | | 2000 | 6.324 | | 1999 | 5.3 | | 1998 | 5.1 | | 1997 | 5 | | 1996 | 4.4 | | 1995 | 3.7 | | 1994 | 4.1 | | 1993 | 4.5 | | 1992 | 4 | | 1991 | 3.6 | | 1990 | 3.9 | | 1989 | 3.7 | | 1988 | 4 | | 1987 | 4.4 | | 1986 | 6.8 | | 1985 | 8.1 | | 1983 | 8.5 |
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