| Year | Value | | 2022 | 2.5 | | 2021 | 2.8 | | 2020 | 4 | | 2019 | 3.4 | | 2018 | 2.9 | | 2017 | 2.4 | | 2016 | 2.3 | | 2014 | 2.1 | | 2013 | 2.1 | | 2012 | 2.3 | | 2011 | 2.1 | | 2010 | 3.1 | | 2009 | 4.1 | | 2008 | 4.8 | | 2007 | 5 | | 2006 | 6 | | 2005 | 8 | | 2004 | 9.8 | | 2003 | 12.7 | | 2002 | 24 | | 2001 | 15.4 | | 2000 | 10.7 | | 1999 | 8.9 | | 1998 | 8.3 | | 1997 | 7.8 | | 1996 | 8.3 | | 1995 | 7.4 | | 1994 | 4.4 | | 1993 | 4.9 | | 1992 | 4.1 | | 1991 | 3 | | 1987 | 2.1 | | 1986 | 1.8 | | 1980 | 1 |
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