| Year | Value | | 2022 | 50.303860473231 | | 2021 | 50.307954299687 | | 2020 | 50.310938312618 | | 2019 | 50.320209823503 | | 2018 | 50.342638395298 | | 2017 | 50.36748022424 | | 2016 | 50.38072709144 | | 2015 | 50.383887037676 | | 2014 | 50.38536791595 | | 2013 | 50.394948438258 | | 2012 | 50.414437062185 | | 2011 | 50.433173673187 | | 2010 | 50.440571165229 | | 2009 | 50.440780692528 | | 2008 | 50.440454323385 | | 2007 | 50.437336414783 | | 2006 | 50.434286479777 | | 2005 | 50.438041246445 | | 2004 | 50.444768352441 | | 2003 | 50.448647905008 | | 2002 | 50.452589868758 | | 2001 | 50.465440773779 | | 2000 | 50.486565775997 | | 1999 | 50.502500380102 | | 1998 | 50.503874431196 | | 1997 | 50.503745438171 | | 1996 | 50.510884880165 | | 1995 | 50.507266735845 | | 1994 | 50.492158896586 | | 1993 | 50.475555359907 | | 1992 | 50.448571717132 | | 1991 | 50.427043548808 | | 1990 | 50.425516621275 | | 1989 | 50.41478973702 | | 1988 | 50.390127728205 | | 1987 | 50.373135810344 | | 1986 | 50.366343390627 | | 1985 | 50.353827874887 | | 1984 | 50.329561178147 | | 1983 | 50.299742996316 | | 1982 | 50.267756714952 | | 1981 | 50.236462387288 | | 1980 | 50.205368824687 | | 1979 | 50.171125099322 | | 1978 | 50.12490667784 | | 1977 | 50.068529575918 | | 1976 | 50.012874265521 | | 1975 | 49.962202175184 | | 1974 | 49.921363053269 | | 1973 | 49.88620359524 | | 1972 | 49.845112241481 | | 1971 | 49.831810316191 | | 1970 | 49.865719300176 | | 1969 | 49.869363514396 | | 1968 | 49.832502320455 | | 1967 | 49.804170413146 | | 1966 | 49.774497747022 | | 1965 | 49.731956815053 | | 1964 | 49.678790474713 | | 1963 | 49.618610084178 | | 1962 | 49.551192398165 | | 1961 | 49.481741065458 | | 1960 | 49.420933952064 |
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