| Year | Value | | 2022 | 2.2772318920615 | | 2021 | 2.3488306608974 | | 2020 | 2.6134673238916 | | 2019 | 2.449055311651 | | 2018 | 2.3578260144139 | | 2017 | 2.4328746724736 | | 2016 | 2.4343732932466 | | 2015 | 2.4049247378583 | | 2014 | 2.5250077583076 | | 2013 | 2.5226766127085 | | 2012 | 2.5896707305629 | | 2011 | 2.6592842266075 | | 2010 | 2.8212281162716 | | 2009 | 2.998103347844 | | 2008 | 2.6451121819889 | | 2007 | 2.5363008182923 | | 2006 | 2.7002718546204 | | 2005 | 2.8997667976268 | | 2004 | 2.8540580353522 | | 2003 | 2.7458349331975 | | 2002 | 2.8630271825825 | | 2001 | 2.9470461015767 | | 2000 | 3.0414995656819 | | 1999 | 3.0867477942053 | | 1998 | 2.9631750641433 | | 1997 | 2.9312613859959 | | 1996 | 2.8903231913579 | | 1995 | 2.9852679334688 | | 1994 | 2.9854808408581 | | 1993 | 3.194364854881 | | 1992 | 3.1066999473844 | | 1991 | 3.2130287010326 | | 1990 | 3.295232489977 | | 1989 | 3.6279286049315 | | 1988 | 3.8172057807944 | | 1987 | 4.2379273409073 | | 1986 | 4.1368377577622 | | 1985 | 3.7039859940345 | | 1984 | 3.5568348698903 | | 1983 | 3.4755163357254 | | 1982 | 3.4697384780047 | | 1981 | 3.221144798305 | | 1980 | 3.1406703024884 | | 1979 | 3.2324769708212 | | 1978 | 3.1707964472129 | | 1977 | 3.1885386833597 | | 1976 | 3.4359282507971 | | 1975 | 3.2412718583204 | | 1974 | 3.0307103298446 | | 1973 | 3.0451198108861 | | 1972 | 4.0414324164522 | | 1971 | 3.9573621015142 | | 1970 | 3.4390427359574 | | 1969 | 3.3724205707903 | | 1968 | 3.4128939856781 | | 1967 | 3.4232956264614 | | 1966 | 3.8675632829732 | | 1965 | 3.9745480603378 | | 1964 | 3.7613932757309 | | 1963 | 3.9216922486005 | | 1962 | 2.7775454935695 | | 1961 | 2.2316791644628 | | 1960 | 2.1817432127648 |
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