| Year | Value | | 2022 | 19.101216314611 | | 2021 | 18.81545119815 | | 2020 | 21.558492887631 | | 2019 | 20.113918016531 | | 2018 | 19.580501745161 | | 2017 | 23.158902992552 | | 2016 | 20.420532307712 | | 2015 | 19.675332379337 | | 2014 | 14.711596300724 | | 2013 | 12.697410026233 | | 2012 | 16.009021356632 | | 2011 | 18.698708734666 | | 2010 | 17.547218684468 | | 2009 | 18.859223710765 | | 2008 | 21.454802676502 | | 2007 | 23.986792130471 | | 2006 | 25.080643173072 | | 2005 | 23.884600403029 | | 2004 | 20.184169059844 | | 2003 | 18.809069971091 | | 2002 | 18.693798649187 | | 2001 | 18.788447507205 | | 2000 | 20.222381797016 | | 1999 | 26.690822017558 | | 1998 | 23.912682352493 | | 1997 | 23.52516620645 | | 1996 | 20.436158792048 | | 1995 | 21.923034644033 | | 1994 | 23.762892640733 | | 1993 | 20.055045689977 | | 1992 | 12.616999997785 | | 1991 | 13.819833903363 | | 1990 | 13.735129018948 | | 1989 | 8.822411329532 | | 1988 | 12.959310780334 | | 1987 | 9.2506462890553 | | 1986 | 5.7191205889582 | | 1985 | 6.7965220016745 | | 1984 | 4.3843823856965 | | 1983 | 2.6298519910711 | | 1982 | 2.7310560486087 | | 1981 | 5.6679929184844 | | 1980 | 8.2907639633041 | | 1979 | 13.391280700117 | | 1978 | 13.900417792501 | | 1977 | 15.339919596883 | | 1976 | 11.442875053519 | | 1975 | 8.9590747330961 | | 1974 | 16.099565006598 |
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