| Year | Value | | 2022 | 2.598 | | 2021 | 3.398 | | 2020 | 2.522 | | 2019 | 2.237 | | 2018 | 2.338 | | 2017 | 2.552 | | 2016 | 2.695 | | 2015 | 3.068 | | 2014 | 3.6 | | 2013 | 3.497 | | 2012 | 3.504 | | 2011 | 3.592 | | 2010 | 3.605 | | 2009 | 3.858 | | 2008 | 3.72 | | 2007 | 3.434 | | 2006 | 4.052 | | 2005 | 3.795 | | 2004 | 3.554 | | 2003 | 3.527 | | 2001 | 3.698 | | 2000 | 11.189 | | 1999 | 9.809 | | 1998 | 10.254 | | 1997 | 8.777 | | 1996 | 8.564 | | 1995 | 9.526 | | 1994 | 9.483 | | 1993 | 9.289 | | 1992 | 9.866 | | 1991 | 10.599 | | 1990 | 8.47 | | 1989 | 8.42 | | 1988 | 8.34 | | 1987 | 9.1 | | 1986 | 6.43 | | 1985 | 6.07 | | 1984 | 7.03 | | 1983 | 4.89 | | 1982 | 5.5 | | 1981 | 5.36 | | 1980 | 4.75 | | 1979 | 3.5 | | 1978 | 4 | | 1977 | 4.47 | | 1976 | 4.8 | | 1975 | 3.83 | | 1974 | 3.21 | | 1973 | 4.77 | | 1972 | 5.36 | | 1971 | 5.28 | | 1969 | 6.7 |
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