| Year | Value | | 2021 | 9.1486693511054 | | 2020 | 9.1224113045788 | | 2019 | 9.1236689208963 | | 2018 | 9.2095313430513 | | 2017 | 9.2614534487425 | | 2016 | 9.2531684721588 | | 2015 | 9.3005107717688 | | 2014 | 9.3331835421781 | | 2013 | 9.3411454686296 | | 2012 | 9.3984259608783 | | 2011 | 9.4485256359808 | | 2010 | 9.2509424924252 | | 2009 | 9.3626433129479 | | 2008 | 9.2285361257573 | | 2007 | 9.1693026369229 | | 2006 | 9.2034332241225 | | 2005 | 9.2637849928436 | | 2004 | 9.2251816281654 | | 2003 | 9.0788079190327 | | 2002 | 9.0645580789434 | | 2001 | 9.0431422384662 | | 2000 | 8.9395085493756 | | 1999 | 8.888443247211 | | 1998 | 8.7733433134199 | | 1997 | 8.7359507161056 | | 1996 | 8.6496725795344 | | 1995 | 8.5348595649339 | | 1994 | 8.623048503209 | | 1993 | 8.6638187037811 | | 1992 | 8.7004724670369 | | 1991 | 8.8326555260846 | | 1990 | 8.8670776644116 | | 1989 | 8.8535947626912 | | 1988 | 8.8367093652493 | | 1987 | 8.8624658479216 | | 1986 | 8.8592402031045 | | 1985 | 8.8148165599474 | | 1984 | 8.5178368217148 | | 1983 | 8.137546504023 | | 1982 | 7.9103013409528 | | 1981 | 7.6428702148673 | | 1980 | 7.5861990329981 | | 1979 | 7.5580526998451 | | 1978 | 7.5271779569567 | | 1977 | 7.4389535543891 | | 1976 | 7.389314979712 | | 1975 | 7.3272627158343 | | 1974 | 7.4203265766586 | | 1973 | 7.3697965962057 | | 1972 | 7.4601011881797 | | 1971 | 7.4393685116103 | | 1970 | 7.557941280234 | | 1969 | 7.6087893453316 | | 1968 | 7.7048203068962 | | 1967 | 7.6334873103764 | | 1966 | 7.6308220090922 | | 1965 | 7.5804647605773 | | 1964 | 7.5409952904964 | | 1963 | 7.494809650914 | | 1962 | 7.5010982394049 | | 1961 | 7.4562788644733 |
|