| Year | Value | | 2021 | 8.6827208802977 | | 2020 | 7.6621629526512 | | 2019 | 7.0560497645168 | | 2018 | 7.035734778393 | | 2017 | 7.0088261303228 | | 2016 | 7.0270066883585 | | 2015 | 7.0530258205985 | | 2014 | 7.0916764516209 | | 2013 | 7.154794060563 | | 2012 | 7.2230599922755 | | 2011 | 7.2879501412384 | | 2010 | 7.3548228742423 | | 2009 | 7.3762449061175 | | 2008 | 7.4302406996189 | | 2007 | 7.4234663589334 | | 2006 | 7.4333913337537 | | 2005 | 7.4246911351091 | | 2004 | 7.473218807255 | | 2003 | 7.5052109439218 | | 2002 | 7.5296992582609 | | 2001 | 7.5674025527074 | | 2000 | 7.6121718832902 | | 1999 | 7.6481720890141 | | 1998 | 7.6697994599604 | | 1997 | 7.7533116549956 | | 1996 | 7.8674317640249 | | 1995 | 7.9771300977513 | | 1994 | 8.043508801084 | | 1993 | 8.1307451340424 | | 1992 | 8.1090909597721 | | 1991 | 8.1730422254377 | | 1990 | 8.2638633911685 | | 1989 | 8.28791622507 | | 1988 | 8.4641251523396 | | 1987 | 8.5914294659893 | | 1986 | 8.7531312064691 | | 1985 | 8.945488870884 | | 1984 | 9.113477902061 | | 1983 | 9.2593966474244 | | 1982 | 9.2948417247352 | | 1981 | 9.3649920771941 | | 1980 | 9.5065132154726 | | 1979 | 9.5531790386996 | | 1978 | 9.7059192440297 | | 1977 | 10.042363012912 | | 1976 | 10.348453468286 | | 1975 | 10.487751625154 | | 1974 | 10.686025962081 | | 1973 | 10.766791141008 | | 1972 | 11.159428245848 | | 1971 | 11.168534736935 | | 1970 | 11.450574937255 | | 1969 | 11.734118818645 | | 1968 | 11.91316740917 | | 1967 | 12.32495318302 | | 1966 | 12.634145674165 | | 1965 | 13.205058892947 | | 1964 | 13.496960648718 | | 1963 | 13.175892072417 | | 1962 | 13.368962070312 | | 1961 | 14.901318895893 | | 1960 | 18.985674712252 |
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