| Year | Value | | 2022 | 3.761 | | 2021 | 4.552 | | 2020 | 6.127 | | 2019 | 3.812 | | 2018 | 3.53 | | 2017 | 3.836 | | 2016 | 4.597 | | 2015 | 3.616 | | 2014 | 3.48 | | 2013 | 3.083 | | 2012 | 3.235 | | 2011 | 3.458 | | 2010 | 4.088 | | 2009 | 4.608 | | 2008 | 3.917 | | 2007 | 3.142 | | 2006 | 3.55 | | 2005 | 3.779 | | 2004 | 5.002 | | 2003 | 5.662 | | 2001 | 4.252 | | 2000 | 4.8 | | 1999 | 13.96 | | 1998 | 11.49 | | 1997 | 9.17 | | 1996 | 10.38 | | 1995 | 6.7 | | 1994 | 6.68 | | 1993 | 7.95 | | 1992 | 8.56 | | 1991 | 8.19 | | 1990 | 2.72 | | 1989 | 7.71 | | 1988 | 6.99 | | 1987 | 7.24 | | 1977 | 5.8 | | 1975 | 5.3 | | 1974 | 4.4 |
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