| Year | Value | | 2021 | 7.881472920479 | | 2020 | 7.8798677325115 | | 2019 | 7.8798677325115 | | 2018 | 7.8798677325115 | | 2017 | 7.8766573565765 | | 2016 | 7.8766573565765 | | 2015 | 7.8766573565765 | | 2014 | 7.8766573565765 | | 2013 | 7.8429484092587 | | 2012 | 7.8333172814537 | | 2011 | 7.8252913416161 | | 2010 | 7.8766573565765 | | 2009 | 7.9232078076343 | | 2008 | 7.9296285595043 | | 2007 | 7.9296285595043 | | 2006 | 7.9296285595043 | | 2005 | 7.9376544993419 | | 2004 | 7.9376544993419 | | 2003 | 7.9376544993419 | | 2002 | 7.9488908151145 | | 2001 | 7.9488908151145 | | 2000 | 7.9488908151145 | | 1999 | 7.9456804391794 | | 1998 | 7.9456804391794 | | 1997 | 7.9456804391794 | | 1996 | 7.9456804391794 | | 1995 | 7.9456804391794 | | 1994 | 7.9617323188545 | | 1993 | 7.9617323188545 | | 1992 | 7.9617323188545 | | 1991 | 7.9585219429195 | | 1990 | 7.9553115669845 | | 1989 | 7.9553115669845 | | 1988 | 7.9553115669845 | | 1987 | 7.953706379017 | | 1986 | 7.9521011910495 | | 1985 | 7.950496003082 | | 1984 | 7.9488908151145 | | 1983 | 7.9456804391794 | | 1982 | 7.9456804391794 | | 1981 | 7.9376544993419 | | 1980 | 7.9376544993419 | | 1979 | 7.9135766798292 | | 1978 | 7.8975248001541 | | 1977 | 7.881472920479 | | 1976 | 7.8654210408039 | | 1975 | 7.8493691611288 | | 1974 | 7.8172654017785 | | 1973 | 7.8012135221034 | | 1972 | 7.7851616424283 | | 1971 | 7.7691097627532 | | 1970 | 7.7530578830781 | | 1969 | 7.737006003403 | | 1968 | 7.7209541237279 | | 1967 | 7.7049022440528 | | 1966 | 7.6888503643777 | | 1965 | 7.6727984847026 | | 1964 | 7.6567466050275 | | 1963 | 7.6342739734823 | | 1962 | 7.6214324697422 | | 1961 | 7.6053805900671 |
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