| Year | Value | | 2022 | 3.524 | | 2021 | 4.21 | | 2020 | 3.819 | | 2019 | 3.38 | | 2018 | 3.831 | | 2017 | 4.839 | | 2016 | 6.006 | | 2015 | 6.873 | | 2014 | 7.416 | | 2013 | 7.243 | | 2012 | 5.821 | | 2011 | 4.977 | | 2010 | 4.989 | | 2009 | 4.347 | | 2008 | 3.654 | | 2007 | 4.153 | | 2006 | 5.002 | | 2005 | 5.873 | | 2004 | 4.646 | | 2003 | 3.593 | | 2002 | 2.554 | | 2001 | 2.119 | | 2000 | 2.725 | | 1999 | 3.623 | | 1998 | 4.394 | | 1997 | 5.511 | | 1996 | 6.421 | | 1995 | 7.158 | | 1994 | 7.159 | | 1993 | 6.287 | | 1992 | 5.555 | | 1991 | 7.285 | | 1990 | 7.671 | | 1989 | 8.704 | | 1988 | 9.369 | | 1987 | 9.898 | | 1985 | 10.48 | | 1983 | 11.851 | | 1981 | 7.92 | | 1979 | 6.16 | | 1977 | 5.88 | | 1975 | 6 |
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