| Year | Value | | 2022 | 19.873859964665 | | 2021 | 9.4962105612495 | | 2020 | 9.7399931389815 | | 2019 | 10.578361800456 | | 2018 | 5.0780572586892 | | 2017 | 4.0853736803261 | | 2016 | 3.7651191635658 | | 2015 | 2.5293281725423 | | 2014 | 7.1893840284703 | | 2013 | 7.6921561189957 | | 2012 | 9.6823518605568 | | 2011 | 11.916092711628 | | 2010 | 12.938870563489 | | 2009 | 13.647765063976 | | 2008 | 20.286121092955 | | 2007 | 7.5986844105077 | | 2006 | 7.9210844005879 | | 2005 | 9.0633273703042 | | 2004 | 7.4446246934274 | | 2003 | 2.9141347005948 | | 2002 | 3.2903447261315 | | 2001 | 3.1482614459062 | | 2000 | 4.3666645129168 | | 1999 | 4.1426371808217 | | 1998 | 6.2280041542454 | | 1997 | 11.375492886512 | | 1996 | 10.3738085885 | | 1995 | 12.343578517051 | | 1994 | 12.368194393695 | | 1993 | 9.9736647602921 | | 1992 | 9.5090414619015 | | 1991 | 11.791270335142 | | 1990 | 9.0521315527978 | | 1989 | 7.8442647374004 | | 1988 | 8.8379370181777 | | 1987 | 4.6812185462021 | | 1986 | 3.506414247582 | | 1985 | 5.6148392179322 | | 1984 | 6.0871667357372 | | 1983 | 6.362033499853 | | 1982 | 5.9035287843586 | | 1981 | 11.879913592528 | | 1980 | 11.938230910709 | | 1979 | 8.2670469761757 | | 1978 | 6.1386926674274 | | 1977 | 10.132967687406 | | 1976 | 7.1583237311846 | | 1975 | 20.9045094574 | | 1974 | 26.663034853749 | | 1973 | 23.070084025084 | | 1972 | 5.1832376454914 | | 1971 | 4.7306914823227 | | 1970 | 5.3498409098302 | | 1969 | 3.1869867957369 | | 1968 | 0.1706273484381 | | 1967 | 6.8113996342869 | | 1966 | 7.2276215978774 | | 1965 | 5.5686354793638 | | 1964 | 4.1795868814532 | | 1963 | 1.456488447975 | | 1962 | 0.5164622336985 | | 1961 | 1.6404199475061 | | 1960 | 6.9473684210529 |
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