| Year | Value | | 2021 | 49.032136001403 | | 2020 | 49.043277617942 | | 2019 | 49.054958771395 | | 2018 | 49.071859878559 | | 2017 | 49.083556264658 | | 2016 | 48.883941933888 | | 2015 | 48.810026149498 | | 2014 | 48.702398931186 | | 2013 | 48.740240939705 | | 2012 | 48.727035780423 | | 2011 | 48.741832836308 | | 2010 | 48.69532827948 | | 2009 | 48.687385179114 | | 2008 | 48.584018240207 | | 2007 | 48.47995457791 | | 2006 | 48.329608431643 | | 2005 | 48.137525659777 | | 2004 | 48.070335178154 | | 2003 | 47.935511864178 | | 2002 | 48.80913805944 | | 2001 | 48.67497946176 | | 2000 | 48.499133976618 | | 1999 | 48.511403185783 | | 1998 | 48.278686610475 | | 1997 | 48.220568372308 | | 1996 | 47.490583178342 | | 1995 | 47.433963103024 | | 1994 | 47.366206078325 | | 1993 | 47.103492611042 | | 1992 | 47.092792211852 | | 1991 | 47.072593705516 | | 1990 | 46.892416690915 | | 1989 | 46.435082969314 | | 1988 | 45.965510142484 | | 1987 | 45.490393646482 | | 1986 | 44.912750912727 | | 1985 | 44.370752832644 | | 1984 | 43.394622172424 | | 1983 | 42.721909309387 | | 1982 | 42.01899927033 | | 1981 | 41.369706511032 | | 1980 | 40.999825436313 | | 1979 | 40.679236187151 | | 1978 | 40.422106761884 | | 1977 | 39.999775786427 | | 1976 | 39.531129853489 | | 1975 | 40.05165170224 | | 1974 | 39.707547793648 | | 1973 | 39.20803925888 | | 1972 | 38.81194532464 | | 1971 | 38.4448850052 | | 1970 | 38.200466909434 | | 1969 | 37.871672877475 | | 1968 | 37.573756780374 | | 1967 | 37.324000718495 | | 1966 | 37.060832977207 | | 1965 | 36.807775413348 | | 1964 | 36.545122167193 | | 1963 | 36.319190457448 | | 1962 | 36.097956284449 | | 1961 | 35.88948703314 |
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