| Year | Value | | 2022 | 50.480455951452 | | 2021 | 50.504360740017 | | 2020 | 50.528595815498 | | 2019 | 50.552068910775 | | 2018 | 50.574088290934 | | 2017 | 50.5971261448 | | 2016 | 50.620617601117 | | 2015 | 50.644824215344 | | 2014 | 50.670957930783 | | 2013 | 50.697855399651 | | 2012 | 50.725882924641 | | 2011 | 50.755194775623 | | 2010 | 50.74783942191 | | 2009 | 50.701246101972 | | 2008 | 50.652897493036 | | 2007 | 50.60172472083 | | 2006 | 50.549660651198 | | 2005 | 50.499707040748 | | 2004 | 50.453728444953 | | 2003 | 50.414239521858 | | 2002 | 50.380569395988 | | 2001 | 50.380157847847 | | 2000 | 50.414579572099 | | 1999 | 50.453116850018 | | 1998 | 50.492666935284 | | 1997 | 50.527917238571 | | 1996 | 50.562111292554 | | 1995 | 50.599287664723 | | 1994 | 50.636238854653 | | 1993 | 50.672243829586 | | 1992 | 50.707701961488 | | 1991 | 50.743653485101 | | 1990 | 50.779947844514 | | 1989 | 50.8167841365 | | 1988 | 50.809362835549 | | 1987 | 50.802804107995 | | 1986 | 50.86786479188 | | 1985 | 50.956561446998 | | 1984 | 51.04659870362 | | 1983 | 51.140523914437 | | 1982 | 51.237751755425 | | 1981 | 51.335195550814 | | 1980 | 51.42940409282 | | 1979 | 51.516125821123 | | 1978 | 51.590766485006 | | 1977 | 51.646125217438 | | 1976 | 51.677015242157 | | 1975 | 51.658547054624 | | 1974 | 51.596636576732 | | 1973 | 51.52210040859 | | 1972 | 51.43594029683 | | 1971 | 51.347793190504 | | 1970 | 51.270748715649 | | 1969 | 51.212058877874 | | 1968 | 51.169698220196 | | 1967 | 51.133786824728 | | 1966 | 51.094173740515 | | 1965 | 51.047677552756 | | 1964 | 50.995309818558 | | 1963 | 50.935617460368 | | 1962 | 50.867609053577 | | 1961 | 50.796785935532 | | 1960 | 50.733998065609 |
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