| Year | Value | | 2022 | 51.036197673872 | | 2021 | 51.074926016106 | | 2020 | 51.091567758234 | | 2019 | 50.8207167923 | | 2018 | 50.855108884119 | | 2017 | 51.093216919183 | | 2016 | 50.939879091593 | | 2015 | 50.780696421736 | | 2014 | 50.615950952917 | | 2013 | 50.441518824757 | | 2012 | 50.333617396494 | | 2011 | 50.298966717278 | | 2010 | 50.264863650706 | | 2009 | 50.232410170674 | | 2008 | 50.201153616655 | | 2007 | 50.169922579809 | | 2006 | 50.137993873679 | | 2005 | 50.104974404673 | | 2004 | 50.07075000263 | | 2003 | 50.038524143988 | | 2002 | 50.04583578295 | | 2001 | 50.090351654166 | | 2000 | 50.135893372119 | | 1999 | 50.182318532375 | | 1998 | 50.227766199681 | | 1997 | 50.270321804105 | | 1996 | 50.313905391794 | | 1995 | 50.361386158371 | | 1994 | 50.411449063833 | | 1993 | 50.462466658369 | | 1992 | 50.514095339853 | | 1991 | 50.536221132757 | | 1990 | 50.517322611214 | | 1989 | 50.4908373921 | | 1988 | 50.469510185246 | | 1987 | 50.451089817286 | | 1986 | 50.435128847065 | | 1985 | 50.421071840408 | | 1984 | 50.408694766929 | | 1983 | 50.397496312817 | | 1982 | 50.386909839041 | | 1981 | 50.377459644011 | | 1980 | 50.36559852784 | | 1979 | 50.349782176991 | | 1978 | 50.334695534401 | | 1977 | 50.320208884417 | | 1976 | 50.305287086641 | | 1975 | 50.290910554058 | | 1974 | 50.277958142933 | | 1973 | 50.266967178862 | | 1972 | 50.257619869417 | | 1971 | 50.25059391739 | | 1970 | 50.237713413899 | | 1969 | 50.218200681401 | | 1968 | 50.199668458507 | | 1967 | 50.182204946494 | | 1966 | 50.166681492851 | | 1965 | 50.15218230502 | | 1964 | 50.140182433718 | | 1963 | 50.130738855781 | | 1962 | 50.121769149501 | | 1961 | 50.112725684976 | | 1960 | 50.129547719366 |
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