| Year | Value | | 2021 | 3.1618047470317 | | 2020 | 3.1618047470317 | | 2019 | 3.1618047470317 | | 2018 | 3.1492524166146 | | 2017 | 3.1367000442114 | | 2016 | 3.1087343250169 | | 2015 | 3.1330442730759 | | 2014 | 3.1361105746219 | | 2013 | 3.1473628523684 | | 2012 | 3.1516874218009 | | 2011 | 3.1497980468061 | | 2010 | 3.1497980468061 | | 2009 | 3.1460192968166 | | 2008 | 3.1443398523768 | | 2007 | 3.1359426301779 | | 2006 | 3.1363624912879 | | 2005 | 3.1535767967956 | | 2004 | 3.1460192968166 | | 2003 | 3.150511810693 | | 2002 | 3.1686917967536 | | 2001 | 3.1838067967117 | | 2000 | 3.2169758243973 | | 1999 | 3.2215942966067 | | 1998 | 3.2165559632873 | | 1997 | 3.211937491078 | | 1996 | 3.1577754078951 | | 1995 | 3.1569356856752 | | 1994 | 3.1393015190575 | | 1993 | 3.1628137412144 | | 1992 | 3.1749897134028 | | 1991 | 3.0481916581995 | | 1990 | 2.9730365195193 | | 1989 | 2.9931898527967 | | 1988 | 2.9814337417182 | | 1987 | 2.9679981862 | | 1986 | 2.9251723529856 | | 1985 | 2.9012402697188 | | 1984 | 2.9033395752685 | | 1983 | 2.7677244367563 | | 1982 | 2.8890642975304 | | 1981 | 2.8907437419702 | | 1980 | 2.8865451308707 | | 1979 | 2.8743691586823 | | 1978 | 2.8936827697398 | | 1977 | 2.8987211030591 | | 1976 | 2.9776549917287 | | 1975 | 2.8739492975724 | | 1974 | 2.747571103479 | | 1973 | 2.7341355479607 | | 1972 | 2.7332958257408 | | 1971 | 2.678713881448 | | 1970 | 2.6232922149353 | | 1969 | 2.6169942982861 | | 1968 | 2.6211929093856 | | 1967 | 2.6455448537624 | | 1966 | 2.6119559649668 | | 1965 | 2.6043984649878 | | 1964 | 2.5745883261817 | | 1963 | 2.603138881658 | | 1962 | 2.6451249926524 | | 1961 | 2.7173411035629 |
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