| Year | Value | | 2021 | 13.452254889056 | | 2020 | 13.901526794067 | | 2019 | 12.776909930176 | | 2018 | 12.712157296325 | | 2017 | 12.687431727488 | | 2016 | 13.123785607826 | | 2015 | 12.345975783405 | | 2014 | 10.242855044729 | | 2013 | 9.883610121992 | | 2012 | 9.3524110630474 | | 2011 | 9.6170721475979 | | 2010 | 9.675317451851 | | 2009 | 9.2346048606448 | | 2008 | 9.5787715286191 | | 2000 | 9.1322954138032 | | 1999 | 8.185176466678 | | 1998 | 7.70264913995 | | 1997 | 8.5922406467086 | | 1996 | 8.3143893039173 | | 1995 | 8.031618305759 | | 1994 | 8.0960591853786 | | 1993 | 8.5671324784273 | | 1992 | 11.602435342957 | | 1991 | 11.594492605771 | | 1990 | 11.848364661184 | | 1989 | 12.139211221392 | | 1988 | 11.411961976225 | | 1987 | 12.747574365303 | | 1986 | 11.864998883777 | | 1985 | 12.312578626034 | | 1984 | 12.623273804501 | | 1983 | 12.999394459093 | | 1982 | 11.217872452694 | | 1981 | 10.545842036971 | | 1980 | 11.423850498887 | | 1979 | 11.342894393742 | | 1978 | 10.74026529739 | | 1977 | 9.9513250405625 | | 1976 | 9.0444930707513 | | 1975 | 9.1818181818182 | | 1974 | 8 | | 1973 | 7.0911722141823 | | 1972 | 7.6106194690265 |
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