| Year | Value | | 2022 | 50.989022915079 | | 2021 | 51.016213742483 | | 2020 | 51.042189977157 | | 2019 | 51.067429557907 | | 2018 | 51.094758906712 | | 2017 | 51.123205084597 | | 2016 | 51.152178366145 | | 2015 | 51.181485066722 | | 2014 | 51.211394540034 | | 2013 | 51.241223855244 | | 2012 | 51.198188835166 | | 2011 | 51.073235744693 | | 2010 | 50.930365926092 | | 2009 | 50.769935496609 | | 2008 | 50.594710849723 | | 2007 | 50.471728627294 | | 2006 | 50.404952049978 | | 2005 | 50.329210320148 | | 2004 | 50.245827783104 | | 2003 | 50.157179590515 | | 2002 | 50.064872171168 | | 2001 | 49.963122184152 | | 2000 | 49.910853531517 | | 1999 | 49.917519235018 | | 1998 | 49.922975917281 | | 1997 | 49.929194719754 | | 1996 | 49.938817926101 | | 1995 | 49.950826373196 | | 1994 | 49.964776124303 | | 1993 | 49.981135988957 | | 1992 | 49.99839350099 | | 1991 | 50.01476066906 | | 1990 | 50.029286744396 | | 1989 | 50.040612979786 | | 1988 | 50.050764830545 | | 1987 | 50.061747336167 | | 1986 | 50.0721631538 | | 1985 | 50.081631367519 | | 1984 | 50.089787252864 | | 1983 | 50.096972502085 | | 1982 | 50.103357750455 | | 1981 | 50.110814874392 | | 1980 | 50.119997823048 | | 1979 | 50.13002950433 | | 1978 | 50.140597811485 | | 1977 | 50.151122578329 | | 1976 | 50.160926771238 | | 1975 | 50.168607133095 | | 1974 | 50.175584014578 | | 1973 | 50.183320595947 | | 1972 | 50.192214716429 | | 1971 | 50.202921060119 | | 1970 | 50.216361105616 | | 1969 | 50.233277493681 | | 1968 | 50.252794177924 | | 1967 | 50.274733851581 | | 1966 | 50.29893525365 | | 1965 | 50.323781276505 | | 1964 | 50.348890523869 | | 1963 | 50.375072699141 | | 1962 | 50.402624111394 | | 1961 | 50.431897389118 | | 1960 | 50.46178909882 |
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