| Year | Value | | 2021 | 8.3565645981294 | | 2020 | 8.3565645981294 | | 2019 | 8.3565645981294 | | 2018 | 8.3565645981294 | | 2017 | 8.3565645981294 | | 2016 | 8.3565645981294 | | 2015 | 8.3565645981294 | | 2014 | 8.3565645981294 | | 2013 | 8.3565645981294 | | 2012 | 8.3104513525051 | | 2011 | 8.2643381068809 | | 2010 | 8.2182248612566 | | 2009 | 8.1985912213296 | | 2008 | 8.1789575814026 | | 2007 | 8.1593239414755 | | 2006 | 8.1396903015485 | | 2005 | 8.1200566616214 | | 2004 | 8.1004230216944 | | 2003 | 8.0807893817674 | | 2002 | 8.0611557418403 | | 2001 | 8.0415221019133 | | 2000 | 8.0218884619863 | | 1999 | 7.9979741529864 | | 1998 | 7.9740598439865 | | 1997 | 7.9501455349866 | | 1996 | 7.9262312259867 | | 1995 | 7.9023169169868 | | 1994 | 7.878402607987 | | 1993 | 7.8544882989871 | | 1992 | 7.8305739899872 | | 1991 | 7.8066596809873 | | 1990 | 7.7827453719874 | | 1989 | 7.7749835060349 | | 1988 | 7.7672216400823 | | 1987 | 7.7594597741297 | | 1986 | 7.7516979081771 | | 1985 | 7.7439360422245 | | 1984 | 7.736174176272 | | 1983 | 7.7284123103194 | | 1982 | 7.7206504443668 | | 1981 | 7.7128885784143 | | 1980 | 7.7051267124617 | | 1979 | 7.6973648465091 | | 1978 | 7.6896029805565 | | 1977 | 7.6042224550782 | | 1976 | 7.5149609966236 | | 1975 | 7.3791283424535 | | 1974 | 7.3053906159041 | | 1973 | 7.2471766212597 | | 1972 | 7.169557961734 | | 1971 | 7.1113439670897 | | 1970 | 7.0337253075639 | | 1969 | 6.9755113129196 | | 1968 | 6.9172973182753 | | 1967 | 6.859083323631 | | 1966 | 6.8008693289867 | | 1965 | 6.7426553343424 | | 1964 | 6.6844413396981 | | 1963 | 6.6262273450538 | | 1962 | 6.5680133504094 | | 1961 | 6.5097993557651 |
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