| Year | Value | | 2022 | 3.6920744951686 | | 2021 | 3.8631706100809 | | 2020 | 3.0621032520071 | | 2019 | 2.6223987240651 | | 2018 | 2.7164530727059 | | 2017 | 2.5634899068124 | | 2016 | 2.5721202244942 | | 2015 | 2.4633032890439 | | 2014 | 2.3522876675734 | | 2013 | 2.367772009446 | | 2012 | 2.4437755868329 | | 2011 | 2.5222787656329 | | 2010 | 2.7502182623658 | | 2009 | 3.2248411378296 | | 2008 | 2.9830523446713 | | 2007 | 2.6793059214126 | | 2006 | 2.783418870265 | | 2005 | 2.8365962771119 | | 2004 | 2.6056725297121 | | 2003 | 2.4938947524986 | | 2002 | 3.0773745753062 | | 2001 | 3.2514128562282 | | 2000 | 3.4653553200888 | | 1999 | 3.3604699446527 | | 1998 | 3.5338688153516 | | 1997 | 3.3658688903991 | | 1996 | 3.3119132818209 | | 1995 | 3.1731901964484 | | 1994 | 3.5819723287729 | | 1993 | 3.600124695617 | | 1992 | 3.6298008904235 | | 1991 | 3.484865109943 | | 1990 | 3.7982603270708 | | 1989 | 3.7658694645888 | | 1988 | 4.1982322306169 | | 1987 | 4.2555204484193 | | 1986 | 4.1280151221131 | | 1985 | 4.6925669384052 | | 1984 | 4.8354443369396 | | 1983 | 5.1724766627771 | | 1982 | 5.6265934582285 | | 1981 | 5.7357013089349 | | 1980 | 4.6608059951787 | | 1979 | 5.181248328693 | | 1978 | 5.5754445124942 | | 1977 | 5.9185356544211 | | 1976 | 5.8292273841529 | | 1975 | 5.8473216222078 | | 1974 | 4.8244688346459 | | 1973 | 3.5177090640908 | | 1972 | 3.9565251449162 | | 1971 | 4.1191991730591 | | 1970 | 4.1973021944836 | | 1969 | 4.788786060059 | | 1968 | 4.6901802664126 | | 1967 | 4.345707667588 | | 1966 | 3.5846873876591 | | 1965 | 3.4984649205928 | | 1964 | 3.5735621103211 | | 1963 | 3.8244846396228 | | 1962 | 4.0484297122829 | | 1961 | 4.2435700002298 | | 1960 | 4.8573029595344 |
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