| Year | Value | | 2021 | 8.9486359421662 | | 2020 | 8.9321899112113 | | 2019 | 8.9027072795867 | | 2018 | 8.9468249620414 | | 2017 | 8.9653887472421 | | 2016 | 8.9749782258976 | | 2015 | 8.9818383304603 | | 2014 | 8.9718952391384 | | 2013 | 8.9921140345068 | | 2012 | 9.0048288789248 | | 2011 | 9.0289155175352 | | 2010 | 9.0262590715682 | | 2009 | 9.0625663480839 | | 2008 | 9.043945949614 | | 2007 | 9.0083896501508 | | 2006 | 9.0155516257694 | | 2005 | 8.9599151181553 | | 2004 | 8.9617669269791 | | 2003 | 8.9906603826929 | | 2002 | 8.993242597252 | | 2001 | 9.0259472884054 | | 2000 | 9.0480040319229 | | 1999 | 9.0795681775417 | | 1998 | 9.1033227765108 | | 1997 | 9.1179988053273 | | 1996 | 9.0835988633608 | | 1995 | 9.1419808650698 | | 1994 | 9.2340086671544 | | 1993 | 9.3096100789771 | | 1992 | 9.4087319768117 | | 1991 | 9.8734612275854 | | 1990 | 9.8890806122876 | | 1989 | 9.8845196501688 | | 1988 | 9.9104920523752 | | 1987 | 9.9459725037051 | | 1986 | 10.001135620512 | | 1985 | 10.034284220256 | | 1984 | 9.7292407097398 | | 1983 | 9.4134876537256 | | 1982 | 9.1423395722765 | | 1981 | 8.8976077169704 | | 1980 | 8.8228691177577 | | 1979 | 8.7178193469519 | | 1978 | 8.6221242890181 | | 1977 | 8.5013198502692 | | 1976 | 8.3572521708921 | | 1975 | 8.2230096737117 | | 1974 | 8.218422135652 | | 1973 | 8.1212688088875 | | 1972 | 8.0574462587067 | | 1971 | 8.0029197901 | | 1970 | 7.9703734903112 | | 1969 | 7.9588673940034 | | 1968 | 7.9586881222639 | | 1967 | 7.9449592986453 | | 1966 | 7.9431605247726 | | 1965 | 7.9314275124342 | | 1964 | 7.9203635389975 | | 1963 | 7.8939714280679 | | 1962 | 7.8690906102037 | | 1961 | 7.851106505363 |
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