| Year | Value | | 2021 | 8.4227823944693 | | 2020 | 7.910489242132 | | 2019 | 7.2803335557319 | | 2018 | 7.2761126770267 | | 2017 | 7.2616421552627 | | 2016 | 7.281421971654 | | 2015 | 7.2732293664875 | | 2014 | 7.2847681649174 | | 2013 | 7.2938261362293 | | 2012 | 7.3301143329591 | | 2011 | 7.3653285743467 | | 2010 | 7.3928392150367 | | 2009 | 7.3747794598076 | | 2008 | 7.4070880891692 | | 2007 | 7.3569961865805 | | 2006 | 7.3206287335601 | | 2005 | 7.228237357715 | | 2004 | 7.2374312495036 | | 2003 | 7.1925981531199 | | 2002 | 7.1694889915833 | | 2001 | 7.1382022908892 | | 2000 | 7.1458109724634 | | 1999 | 7.121540361812 | | 1998 | 7.0809189919916 | | 1997 | 7.1239833042435 | | 1996 | 7.2011133507621 | | 1995 | 7.2812018429921 | | 1994 | 7.2951453793843 | | 1993 | 7.3360886668894 | | 1992 | 7.2020014551421 | | 1991 | 7.2121018295281 | | 1990 | 7.2270043956919 | | 1989 | 7.1622531241133 | | 1988 | 7.2792654127722 | | 1987 | 7.3277465932938 | | 1986 | 7.4506203768419 | | 1985 | 7.5701837804754 | | 1984 | 7.6860449196427 | | 1983 | 7.7587527841615 | | 1982 | 7.6525273522098 | | 1981 | 7.6054168227724 | | 1980 | 7.6806522160491 | | 1979 | 7.6447347967286 | | 1978 | 7.7186249566324 | | 1977 | 8.1074742545369 | | 1976 | 8.3955206402152 | | 1975 | 8.4787622169058 | | 1974 | 8.5309292884618 | | 1973 | 8.4410864450508 | | 1972 | 8.8415954243251 | | 1971 | 8.763222087459 | | 1970 | 9.0589978048277 | | 1969 | 9.3645870333132 | | 1968 | 9.5129524484135 | | 1967 | 9.7130678744227 | | 1966 | 10.100335803567 | | 1965 | 10.856982728486 | | 1964 | 11.634248550696 | | 1963 | 10.989610696847 | | 1962 | 11.113912708559 | | 1961 | 13.494394244402 | | 1960 | 19.856542206194 |
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