| Year | Value | | 2021 | 13.896554376818 | | 2020 | 14.007847590357 | | 2019 | 14.007847590357 | | 2018 | 14.007847590357 | | 2017 | 14.007847590357 | | 2016 | 13.421968033346 | | 2015 | 12.676303142605 | | 2014 | 12.516517808874 | | 2013 | 12.500539275501 | | 2012 | 12.484560742128 | | 2011 | 12.468582208755 | | 2010 | 12.452603675382 | | 2009 | 12.436625142009 | | 2008 | 12.420646608636 | | 2007 | 12.404668075263 | | 2006 | 12.38868954189 | | 2005 | 12.372711008517 | | 2004 | 12.356732475144 | | 2003 | 11.997215474251 | | 2002 | 11.637698473358 | | 2001 | 11.278181472464 | | 2000 | 10.918664471571 | | 1999 | 10.874545283925 | | 1998 | 10.322537909107 | | 1997 | 10.212136434143 | | 1996 | 9.9035643116192 | | 1995 | 9.5729118941029 | | 1994 | 9.4536783011421 | | 1993 | 10.007341698085 | | 1992 | 9.9913334842153 | | 1991 | 9.9808453440938 | | 1990 | 11.1798053622 | | 1989 | 11.514873838715 | | 1988 | 11.678268021661 | | 1987 | 11.685996124908 | | 1986 | 11.138404809088 | | 1985 | 10.764143808961 | | 1984 | 9.9477249016047 | | 1983 | 9.9361327467335 | | 1982 | 9.9361327467335 | | 1981 | 9.9361327467335 | | 1980 | 9.9361327467335 | | 1979 | 9.9361327467335 | | 1978 | 9.9361327467335 | | 1977 | 9.9361327467335 | | 1976 | 9.9361327467335 | | 1975 | 9.9361327467335 | | 1974 | 9.9361327467335 | | 1973 | 9.9361327467335 | | 1972 | 9.9361327467335 | | 1971 | 9.9361327467335 | | 1970 | 9.9361327467335 | | 1969 | 9.9361327467335 | | 1968 | 9.9361327467335 | | 1967 | 9.9361327467335 | | 1966 | 9.9361327467335 | | 1965 | 9.9361327467335 | | 1964 | 9.9361327467335 | | 1963 | 9.9361327467335 | | 1962 | 9.9361327467335 | | 1961 | 9.9361327467335 |
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