| Year | Value | | 2022 | 2.5585775880018 | | 2021 | 2.4760780571322 | | 2020 | 2.5940071041645 | | 2019 | 2.402701726962 | | 2018 | 2.3381838122277 | | 2017 | 2.3405909264043 | | 2016 | 2.371295033234 | | 2015 | 2.4461417420587 | | 2014 | 2.4429591392078 | | 2013 | 2.5220638703677 | | 2012 | 2.6326611488177 | | 2011 | 2.6958223743536 | | 2010 | 2.7886992752841 | | 2009 | 2.8237672071956 | | 2008 | 2.5687917718619 | | 2007 | 2.4754017940337 | | 2006 | 2.4988066515875 | | 2005 | 2.5086142009167 | | 2004 | 2.4990081559652 | | 2003 | 2.4815578690297 | | 2002 | 2.398616074236 | | 2001 | 2.2851190683031 | | 2000 | 2.2558522070719 | | 1999 | 2.2509269496865 | | 1998 | 2.3338538183585 | | 1997 | 2.3723930417212 | | 1996 | 2.3578738335969 | | 1995 | 2.4208829209492 | | 1994 | 2.6338682582102 | | 1993 | 2.8361468973841 | | 1992 | 3.0042215783577 | | 1991 | 3.0963299071665 | | 1990 | 3.4183875856428 | | 1989 | 3.5287060560153 | | 1988 | 3.5929346681106 | | 1987 | 3.8591711474694 | | 1986 | 4.0162069267094 | | 1985 | 4.4281193682023 | | 1984 | 4.3703814650611 | | 1983 | 4.3475636729764 | | 1982 | 4.5676691164981 | | 1981 | 3.9535950118928 | | 1980 | 3.64794373722 | | 1979 | 3.5706471680436 | | 1978 | 3.5485695729897 | | 1977 | 3.7237791259187 | | 1976 | 3.5996545083831 | | 1975 | 3.7918489337503 | | 1974 | 3.896568993374 | | 1973 | 3.9188609743568 | | 1972 | 4.381077660485 | | 1971 | 4.6729338962564 | | 1970 | 5.2322027660335 | | 1969 | 5.9895837135025 | | 1968 | 6.4883875432273 | | 1967 | 6.6144037357509 | | 1966 | 6.1425701573749 | | 1965 | 5.7293376391689 | | 1964 | 6.0400550428494 | | 1963 | 6.5746301539541 | | 1962 | 6.9943886429258 | | 1961 | 6.9253912719074 | | 1960 | 6.9582090943471 |
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