| Year | Value | | 2021 | 13.084267531033 | | 2020 | 13.07258216695 | | 2019 | 13.011859141071 | | 2018 | 13.033205821618 | | 2017 | 13.037964735439 | | 2016 | 13.031360569666 | | 2015 | 13.008213356986 | | 2014 | 12.95474827059 | | 2013 | 12.951513451869 | | 2012 | 12.963942317834 | | 2011 | 12.938213104971 | | 2010 | 12.923280538734 | | 2009 | 12.840599223781 | | 2008 | 12.876814199459 | | 2007 | 12.984468974533 | | 2006 | 12.954431360157 | | 2005 | 12.913865223905 | | 2004 | 12.859648903624 | | 2003 | 12.806342372121 | | 2002 | 12.715824180162 | | 2001 | 12.67527911498 | | 2000 | 12.624172899863 | | 1999 | 12.677773691092 | | 1998 | 12.646572796384 | | 1997 | 12.609144508039 | | 1996 | 12.545694763013 | | 1995 | 12.537887947358 | | 1994 | 12.500932884835 | | 1993 | 12.512174900989 | | 1992 | 12.443853622711 | | 1991 | 12.631369776398 | | 1990 | 12.620637890177 | | 1989 | 12.625582758831 | | 1988 | 12.564729455869 | | 1987 | 12.568233805971 | | 1986 | 12.436248693482 | | 1985 | 12.426974650568 | | 1984 | 12.353621123689 | | 1983 | 12.237887792484 | | 1982 | 12.318177369621 | | 1981 | 12.249548991958 | | 1980 | 12.248654172259 | | 1979 | 12.298987793561 | | 1978 | 12.234514569929 | | 1977 | 12.181854216232 | | 1976 | 12.172005828636 | | 1975 | 12.160959670828 | | 1974 | 12.151631551965 | | 1973 | 12.160084959669 | | 1972 | 12.08822755741 | | 1971 | 11.981338085439 | | 1970 | 12.003830511359 | | 1969 | 12.009682578006 | | 1968 | 12.008037169347 | | 1967 | 11.960134045545 | | 1966 | 11.894317699167 | | 1965 | 11.842037167422 | | 1964 | 11.757841916773 | | 1963 | 11.709038475025 | | 1962 | 11.629593177676 | | 1961 | 11.563155922371 |
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