| Year | Value | | 2021 | 9.6758089368259 | | 2020 | 9.6515665331279 | | 2019 | 9.6273240677966 | | 2018 | 9.6030816640986 | | 2017 | 9.3275808936826 | | 2016 | 9.0172892261598 | | 2015 | 9.0172892261598 | | 2014 | 9.0172892261598 | | 2013 | 9.0172892261598 | | 2012 | 9.2327905891598 | | 2011 | 9.3408482526585 | | 2010 | 9.4489059161571 | | 2009 | 9.5569635796557 | | 2008 | 9.5440703357155 | | 2007 | 10.356958667944 | | 2006 | 10.253812716422 | | 2005 | 10.150666764901 | | 2004 | 10.047520813379 | | 2003 | 9.9443748618581 | | 2002 | 9.8412289103367 | | 2001 | 9.7380829588153 | | 2000 | 9.1627986934846 | | 1999 | 10.030329821459 | | 1998 | 10.337311820035 | | 1997 | 10.131633880989 | | 1996 | 10.496942459294 | | 1995 | 10.675605982465 | | 1994 | 10.64429381861 | | 1993 | 10.641223998625 | | 1992 | 10.418355067659 | | 1991 | 10.241533436479 | | 1990 | 9.3261131167268 | | 1989 | 9.4550455561286 | | 1988 | 9.5655590756158 | | 1987 | 9.4550455561286 | | 1986 | 9.3015545568408 | | 1985 | 9.148063557553 | | 1984 | 8.9945725582652 | | 1983 | 8.8410815589774 | | 1982 | 8.6820648837152 | | 1981 | 8.3192121613989 | | 1980 | 7.9698666470198 | | 1979 | 9.3285689727154 | | 1978 | 8.7133770475699 | | 1977 | 9.1904270733564 | | 1976 | 9.412068076328 | | 1975 | 9.7313293548466 | | 1974 | 10.050590633365 | | 1973 | 10.376605515853 | | 1972 | 9.9627937817726 | | 1971 | 9.566173039613 | | 1970 | 9.3015545568408 | | 1969 | 9.2831356369263 | | 1968 | 9.2708563569832 | | 1967 | 9.2401581571257 | | 1966 | 9.2278788771826 | | 1965 | 9.2217392372111 | | 1964 | 9.2155995972396 | | 1963 | 9.2094599572681 | | 1962 | 9.2033203172966 | | 1961 | 9.1971806773251 |
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