| Year | Value | | 2021 | 10.331541449111 | | 2020 | 10.324854034312 | | 2019 | 9.9796805473392 | | 2018 | 10.236888808868 | | 2017 | 10.4205355076 | | 2016 | 11.188044960004 | | 2015 | 11.341855500399 | | 2014 | 11.399984567504 | | 2013 | 11.569742020114 | | 2012 | 11.60112142802 | | 2011 | 11.685485737802 | | 2010 | 11.797114123306 | | 2009 | 11.905656009671 | | 2008 | 11.988991486406 | | 2007 | 12.098562205818 | | 2006 | 12.018313228221 | | 2005 | 11.983847321176 | | 2004 | 11.941665166285 | | 2003 | 12.166979603385 | | 2002 | 11.926232670593 | | 2001 | 11.909256925332 | | 2000 | 11.78271046066 | | 1999 | 11.78939787546 | | 1998 | 11.744643637954 | | 1997 | 11.720980477893 | | 1996 | 11.686000154325 | | 1995 | 11.605236760205 | | 1994 | 11.599578178451 | | 1993 | 11.590318681036 | | 1992 | 11.537848195684 | | 1991 | 11.534761696546 | | 1990 | 11.167468299082 | | 1989 | 10.625787700301 | | 1988 | 10.132976671211 | | 1987 | 9.8942874045114 | | 1986 | 9.4364566989892 | | 1985 | 9.4099642480517 | | 1984 | 9.3834717971141 | | 1983 | 9.4282260346202 | | 1982 | 9.1540420278299 | | 1981 | 9.0532163893104 | | 1980 | 9.084595797217 | | 1979 | 9.0460145579876 | | 1978 | 9.1488978625994 | | 1977 | 9.1473546130302 | | 1976 | 9.1236914529695 | | 1975 | 9.1113454564161 | | 1974 | 9.0989994598627 | | 1973 | 9.1149463720775 | | 1972 | 9.0969417937704 | | 1971 | 9.0995138763857 | | 1970 | 9.0989994598627 | | 1969 | 9.6555981378122 | | 1968 | 9.6967514596569 | | 1967 | 9.7379047815016 | | 1966 | 9.7842022685769 | | 1965 | 9.8253555904216 | | 1964 | 9.8665089122663 | | 1963 | 9.907662234111 | | 1962 | 9.9488155559557 | | 1961 | 10.034723115306 |
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