| Year | Value | | 2021 | 11.777240841777 | | 2020 | 11.777240841777 | | 2019 | 11.795946999221 | | 2018 | 11.896648480125 | | 2017 | 12.014029618083 | | 2016 | 12.07492211838 | | 2015 | 12.176323987539 | | 2014 | 12.021806853583 | | 2013 | 11.892523364486 | | 2012 | 11.809968847352 | | 2011 | 11.688473520249 | | 2010 | 11.551401869159 | | 2009 | 11.366043613707 | | 2008 | 11.431464174455 | | 2007 | 11.498442367601 | | 2006 | 11.566978193146 | | 2005 | 11.576323987539 | | 2004 | 11.542056074766 | | 2003 | 11.632398753894 | | 2002 | 11.660436137072 | | 2001 | 11.719626168224 | | 2000 | 11.767912772586 | | 1999 | 11.886292834891 | | 1998 | 11.933021806854 | | 1997 | 11.992211838006 | | 1996 | 12.098130841122 | | 1995 | 12.157320872274 | | 1994 | 12.138629283489 | | 1993 | 12.213395638629 | | 1992 | 12.303738317757 | | 1991 | 12.347352024922 | | 1990 | 12.048997772828 | | 1989 | 11.765828825962 | | 1988 | 11.858097359211 | | 1987 | 11.51924912504 | | 1986 | 11.291759465479 | | 1985 | 11.110404072542 | | 1984 | 10.839961819917 | | 1983 | 10.626789691378 | | 1982 | 10.426344257079 | | 1981 | 10.203627107859 | | 1980 | 9.9490932230353 | | 1979 | 9.8456888323258 | | 1978 | 9.6786509704104 | | 1977 | 9.5036589245943 | | 1976 | 9.2809417753739 | | 1975 | 8.9945911549475 | | 1974 | 8.8036907413299 | | 1973 | 8.7082405345212 | | 1972 | 8.6286986955138 | | 1971 | 8.4696150174992 | | 1970 | 8.4616608335985 | | 1969 | 8.358256442889 | | 1968 | 8.2946229716831 | | 1967 | 8.3105313394846 | | 1966 | 8.4218899140948 | | 1965 | 8.4059815462934 | | 1964 | 8.2612153993 | | 1963 | 8.2612153993 | | 1962 | 8.1657651924912 | | 1961 | 8.1339484568883 |
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