| Year | Value | | 2022 | 2.1869169343727 | | 2021 | 2.2688892102897 | | 2020 | 2.3621118392394 | | 2019 | 2.3683883139218 | | 2018 | 2.4175606866867 | | 2017 | 2.3576923293923 | | 2016 | 2.4594025000555 | | 2015 | 2.7149764701354 | | 2014 | 3.5022767504677 | | 2013 | 4.6654185971967 | | 2012 | 4.4884639176917 | | 2011 | 3.4985800882497 | | 2010 | 3.1709332021716 | | 2009 | 3.5987933376422 | | 2008 | 1.9264626553176 | | 2007 | 0.85022191619316 | | 2006 | 0.7184322085238 | | 2005 | 2.9697060646724 | | 2004 | 2.8771584595083 | | 2003 | 3.0513111631183 | | 2002 | 3.2110492891941 | | 2001 | 3.1840173453354 | | 2000 | 3.3362466943399 | | 1999 | 3.2739296689177 | | 1998 | 3.1851533188697 | | 1997 | 3.1452963275892 | | 1996 | 3.2428450597752 | | 1995 | 3.4024505611852 | | 1994 | 4.8171240037014 | | 1993 | 4.8316668481659 | | 1992 | 2.9771145541281 | | 1991 | 1.0590059601607 | | 1990 | 1.2295598240363 | | 1989 | 2.644620268191 | | 1988 | 2.3185363055008 | | 1987 | 2.7473462089413 | | 1986 | 2.4724728098798 | | 1985 | 2.421019314212 | | 1984 | 3.1262980687368 | | 1983 | 2.9694652566891 | | 1982 | 2.8067222728154 | | 1981 | 2.986380721467 | | 1980 | 3.1209874557525 | | 1979 | 3.1792833489743 | | 1978 | 3.3271801242903 | | 1977 | 3.2859980122982 | | 1976 | 3.2328731630656 | | 1975 | 3.2764384779435 | | 1974 | 3.3555236637931 | | 1973 | 3.4131780632672 | | 1972 | 3.4112576366755 | | 1971 | 3.4419531884042 | | 1970 | 3.4699514314915 | | 1969 | 3.4416239942575 | | 1968 | 3.4175639606197 | | 1967 | 3.3862847256463 | | 1966 | 3.3685045099981 | | 1965 | 3.3551913089338 | | 1964 | 3.328636659371 | | 1963 | 3.3384325332068 | | 1962 | 2.9999510676145 | | 1961 | 2.4543747128389 |
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