| Year | Value | | 2022 | 3.695 | | 2021 | 4.812 | | 2020 | 4.169 | | 2019 | 3.731 | | 2018 | 3.917 | | 2017 | 4.14 | | 2016 | 4.723 | | 2015 | 5.176 | | 2014 | 5.789 | | 2013 | 6.137 | | 2012 | 6.762 | | 2011 | 7.139 | | 2010 | 8.48 | | 2009 | 9.534 | | 2008 | 7.704 | | 2007 | 9.379 | | 2006 | 10.707 | | 2005 | 11.342 | | 2004 | 13.029 | | 2003 | 13.505 | | 2002 | 12.888 | | 2001 | 11.812 | | 2000 | 11.102 | | 1999 | 11.183 | | 1998 | 10.749 | | 1997 | 9.751 | | 1996 | 8.455 | | 1995 | 8.782 | | 1994 | 9.934 | | 1993 | 12.741 | | 1992 | 14.08 | | 1991 | 13.387 | | 1990 | 12.241 | | 1985 | 6.33 | | 1984 | 5.69 | | 1983 | 4.32 | | 1982 | 4.81 | | 1981 | 4.92 | | 1980 | 4.59 | | 1979 | 2.78 | | 1978 | 3.42 | | 1976 | 3.47 | | 1974 | 3.05 | | 1973 | 2.64 | | 1972 | 2.7 | | 1971 | 3.46 | | 1970 | 3.82 | | 1969 | 4.47 |
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