| Year | Value | | 2022 | 2.857 | | 2021 | 3.639 | | 2020 | 3.931 | | 2019 | 3.746 | | 2018 | 3.824 | | 2017 | 3.653 | | 2016 | 3.65 | | 2015 | 3.546 | | 2014 | 3.082 | | 2013 | 2.747 | | 2012 | 2.812 | | 2011 | 2.992 | | 2010 | 3.324 | | 2009 | 3.363 | | 2008 | 2.959 | | 2007 | 3.013 | | 2006 | 3.254 | | 2005 | 3.482 | | 2004 | 3.42 | | 2003 | 3.348 | | 2002 | 3.046 | | 2001 | 3.699 | | 2000 | 4.063 | | 1999 | 6.342 | | 1998 | 6.963 | | 1997 | 2.608 | | 1996 | 2.048 | | 1995 | 2.063 | | 1994 | 2.476 | | 1993 | 2.878 | | 1992 | 2.513 | | 1991 | 2.407 | | 1990 | 2.454 | | 1989 | 2.569 | | 1988 | 2.519 | | 1987 | 3.088 | | 1986 | 3.81 | | 1985 | 3.983 | | 1984 | 3.801 | | 1983 | 4.068 | | 1982 | 4.363 | | 1981 | 4.507 | | 1980 | 5.196 | | 1979 | 3.812 | | 1978 | 3.167 | | 1977 | 3.806 | | 1976 | 3.808 | | 1975 | 4.18 | | 1974 | 4.15 | | 1973 | 4.04 | | 1972 | 4.59 | | 1971 | 4.57 | | 1970 | 4.51 | | 1969 | 4.86 |
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