| Year | Value | | 2022 | 3.2812047641229 | | 2021 | 3.2843085242585 | | 2020 | 3.5287242897954 | | 2019 | 3.2724421287115 | | 2018 | 3.1683593895369 | | 2017 | 3.1668570574891 | | 2016 | 3.251948324021 | | 2015 | 3.2980169066876 | | 2014 | 3.4389827649788 | | 2013 | 3.7332605246479 | | 2012 | 4.123657045645 | | 2011 | 4.448255093748 | | 2010 | 4.5440255154284 | | 2009 | 4.5724298626157 | | 2008 | 4.142038379213 | | 2007 | 3.8073218570635 | | 2006 | 3.7869167301068 | | 2005 | 3.8430819723855 | | 2004 | 3.7905595707785 | | 2003 | 3.630302509273 | | 2002 | 3.2959569624416 | | 2001 | 2.9938813158062 | | 2000 | 2.9769188652702 | | 1999 | 2.9643359090824 | | 1998 | 3.0743726809974 | | 1997 | 3.2523730158376 | | 1996 | 3.3991199955717 | | 1995 | 3.6907709231057 | | 1994 | 4.0296023755095 | | 1993 | 4.3877118819534 | | 1992 | 4.7107488614053 | | 1991 | 4.6131123915257 | | 1990 | 5.2738067512609 | | 1989 | 5.5115737274307 | | 1988 | 5.7091764131746 | | 1987 | 6.0633633938441 | | 1986 | 6.2812469348342 | | 1985 | 6.1142196660431 | | 1984 | 5.9020302096521 | | 1983 | 5.9565743051323 | | 1982 | 6.3960921873373 | | 1981 | 5.3023705087403 | | 1980 | 4.8562139191036 | | 1979 | 4.676712052393 | | 1978 | 4.6790726583317 | | 1977 | 4.8455862183349 | | 1976 | 4.849868661752 | | 1975 | 5.2700413144292 | | 1974 | 5.5607170058605 | | 1973 | 5.550208302669 | | 1972 | 6.1524600290799 | | 1971 | 6.5640412773441 | | 1970 | 7.5890269808302 | | 1969 | 8.1752833582234 | | 1968 | 8.7922363867281 | | 1967 | 8.945490406923 | | 1966 | 8.0342203461942 | | 1965 | 7.2691920176427 | | 1964 | 7.7390206320518 | | 1963 | 8.4886322158207 | | 1962 | 8.9838180370578 | | 1961 | 8.8154198787127 | | 1960 | 8.6598813962761 |
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