| Year | Value | | 2022 | 51.983916724158 | | 2021 | 51.782410832202 | | 2020 | 51.604372914509 | | 2019 | 51.412647180225 | | 2018 | 51.283110772308 | | 2017 | 51.196890834754 | | 2016 | 51.069196962894 | | 2015 | 50.956253591547 | | 2014 | 50.856751532925 | | 2013 | 50.772191374608 | | 2012 | 50.703279984077 | | 2011 | 50.632864603114 | | 2010 | 50.592299761951 | | 2009 | 50.573363494796 | | 2008 | 50.560718187692 | | 2007 | 50.551137764363 | | 2006 | 50.543509615392 | | 2005 | 50.537528903749 | | 2004 | 50.534720921127 | | 2003 | 50.52208449492 | | 2002 | 50.505305350906 | | 2001 | 50.471638589226 | | 2000 | 50.397295834744 | | 1999 | 50.25963414552 | | 1998 | 50.112487969605 | | 1997 | 49.961732709177 | | 1996 | 49.810374866482 | | 1995 | 49.656163878347 | | 1994 | 49.504475295381 | | 1993 | 49.36177552916 | | 1992 | 49.219632020801 | | 1991 | 49.072183909316 | | 1990 | 48.890695508264 | | 1989 | 48.673053260731 | | 1988 | 48.444630559709 | | 1987 | 48.218995827047 | | 1986 | 47.99643160831 | | 1985 | 47.773470199899 | | 1984 | 47.558110900291 | | 1983 | 47.353525689482 | | 1982 | 47.157156775005 | | 1981 | 46.841112914714 | | 1980 | 46.531856684117 | | 1979 | 45.920279441858 | | 1978 | 45.287101152714 | | 1977 | 44.649573872991 | | 1976 | 44.007741890554 | | 1975 | 43.358723786233 | | 1974 | 42.705443948716 | | 1973 | 42.047772506404 | | 1972 | 41.386659430591 | | 1971 | 40.817215707553 | | 1970 | 40.341961542313 | | 1969 | 39.812325890257 | | 1968 | 39.263414672292 | | 1967 | 38.713406991025 | | 1966 | 38.168849959051 | | 1965 | 37.629410413501 | | 1964 | 37.096188396122 | | 1963 | 36.549339390135 | | 1962 | 36.000059523133 | | 1961 | 35.457237135992 | | 1960 | 34.923470024169 |
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