| Year | Value | | 2021 | 9.6566533051902 | | 2020 | 9.6300861519486 | | 2019 | 9.6023427446112 | | 2018 | 9.5506635409915 | | 2017 | 9.4851997548342 | | 2016 | 9.4018357599053 | | 2015 | 9.2998193590028 | | 2014 | 9.2415075865799 | | 2013 | 9.1677310603602 | | 2012 | 9.0335052618392 | | 2011 | 8.8501370375862 | | 2010 | 8.7014434074559 | | 2009 | 8.6131400258529 | | 2008 | 8.3894836810618 | | 2007 | 8.2626034807028 | | 2006 | 8.1407542821453 | | 2005 | 8.0873669211005 | | 2004 | 7.943932967384 | | 2003 | 7.9226301292069 | | 2002 | 7.7028790319962 | | 2001 | 7.6291685020378 | | 2000 | 7.5723966674586 | | 1999 | 7.5582010634659 | | 1998 | 7.4523762846239 | | 1997 | 7.4228526983459 | | 1996 | 7.3725498033593 | | 1995 | 7.2673286804285 | | 1994 | 7.0637459202468 | | 1993 | 6.9853699449772 | | 1992 | 6.8475450938143 | | 1991 | 6.7322464548844 | | 1990 | 6.6365947379022 | | 1989 | 6.569016101203 | | 1988 | 6.5317546975092 | | 1987 | 6.4818752275645 | | 1986 | 6.4801815273966 | | 1985 | 6.4507958294835 | | 1984 | 6.3457864190737 | | 1983 | 6.3202962315468 | | 1982 | 6.2898096285245 | | 1981 | 6.2561473376875 | | 1980 | 6.2570788727799 | | 1979 | 6.2399725010841 | | 1978 | 6.2246021720604 | | 1977 | 6.1579550704535 | | 1976 | 6.1438973590599 | | 1975 | 6.1108278632816 | | 1974 | 6.0586863096427 | | 1973 | 6.0704391967205 | | 1972 | 6.058160286439 | | 1971 | 6.0236946555112 | | 1970 | 5.9784743928195 | | 1969 | 5.9607758255863 | | 1968 | 5.9191122127693 | | 1967 | 5.8476404867174 | | 1966 | 5.8170278931537 | | 1965 | 5.7885746941567 | | 1964 | 5.7412373779683 | | 1963 | 5.6998278115709 | | 1962 | 5.6165005859369 | | 1961 | 5.5738631285114 |
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