| Year | Value | | 2022 | 50.668589333536 | | 2021 | 50.658746571266 | | 2020 | 50.659464974935 | | 2019 | 50.669227057351 | | 2018 | 50.687005605507 | | 2017 | 50.707010955871 | | 2016 | 50.729614454594 | | 2015 | 50.755409080203 | | 2014 | 50.787355350838 | | 2013 | 50.828785224638 | | 2012 | 50.87627596585 | | 2011 | 50.925735531196 | | 2010 | 50.970042224757 | | 2009 | 51.00850152476 | | 2008 | 51.032975207191 | | 2007 | 51.077583925101 | | 2006 | 51.109379900647 | | 2005 | 51.09900557493 | | 2004 | 51.094498595738 | | 2003 | 51.092858612264 | | 2002 | 51.093398159931 | | 2001 | 51.094863496531 | | 2000 | 51.093799252488 | | 1999 | 51.086803363482 | | 1998 | 51.073180975998 | | 1997 | 51.055800553489 | | 1996 | 51.025063705247 | | 1995 | 50.981009565793 | | 1994 | 50.938009318334 | | 1993 | 50.895953174739 | | 1992 | 50.85512188009 | | 1991 | 50.817787723373 | | 1990 | 50.780685015039 | | 1989 | 50.741982161643 | | 1988 | 50.701751653801 | | 1987 | 50.659440980518 | | 1986 | 50.579198731408 | | 1985 | 50.515711973239 | | 1984 | 50.53325341471 | | 1983 | 50.574875631481 | | 1982 | 50.615997457671 | | 1981 | 50.656614399576 | | 1980 | 50.69881335993 | | 1979 | 50.744201438071 | | 1978 | 50.793524402181 | | 1977 | 50.855624268503 | | 1976 | 50.971684343061 | | 1975 | 51.195875610518 | | 1974 | 51.522372973934 | | 1973 | 51.897491350624 | | 1972 | 52.299037040143 | | 1971 | 52.733095004241 | | 1970 | 53.203527959972 | | 1969 | 53.713854211268 | | 1968 | 54.264988583325 | | 1967 | 54.849100523112 | | 1966 | 55.192765371277 | | 1965 | 55.284694613006 | | 1964 | 55.394712394287 | | 1963 | 55.517600211583 | | 1962 | 55.650640196462 | | 1961 | 55.792454937935 | | 1960 | 55.943852173512 |
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