| Year | Value | | 2021 | 1.4894321665919 | | 2020 | 1.5596700980161 | | 2019 | 1.6920497928429 | | 2018 | 1.7328369419079 | | 2017 | 1.8942968538544 | | 2016 | 1.8813016241583 | | 2015 | 1.8300318148409 | | 2014 | 1.8956857424622 | | 2013 | 1.8667119055047 | | 2012 | 1.9299609317804 | | 2011 | 1.8442531324919 | | 2010 | 1.8525666691756 | | 2009 | 1.8755416903988 | | 2008 | 1.8735816824999 | | 2007 | 1.8533040291873 | | 2006 | 1.8305358539129 | | 2005 | 1.8137863350692 | | 2004 | 1.8084821560499 | | 2003 | 1.7923466465847 | | 2002 | 1.80020140853 | | 2001 | 1.8100357831363 | | 2000 | 1.8663398450123 | | 1999 | 1.8104689664588 | | 1998 | 1.8244023064147 | | 1997 | 1.8528019408302 | | 1996 | 1.8875821486473 | | 1995 | 1.9255711319391 | | 1994 | 1.9733033238274 | | 1993 | 2.0279870566565 | | 1992 | 2.1032281110248 | | 1991 | 2.2229933342824 | | 1990 | 2.6017445956545 | | 1989 | 2.6234487801349 | | 1988 | 2.6649687773741 | | 1987 | 2.8251230614516 | | 1986 | 2.8272115725262 | | 1985 | 2.8119313398524 | | 1984 | 2.8405490617242 | | 1983 | 2.8497648944258 | | 1982 | 3.1443708339087 | | 1981 | 3.0569976143769 | | 1980 | 3.0566889411647 | | 1979 | 3.086592062694 | | 1978 | 3.083300736304 | | 1977 | 3.1939876828608 | | 1976 | 3.4734377972304 | | 1975 | 3.7250191646573 | | 1974 | 4.1469435152803 | | 1973 | 4.5490246635479 | | 1972 | 4.8191461857764 | | 1971 | 5.1007305673047 | | 1970 | 5.4594525691403 | | 1969 | 5.5217186466937 | | 1968 | 5.7343985499848 | | 1967 | 5.3227503773341 | | 1966 | 5.5717081938139 | | 1965 | 5.8269281258835 | | 1964 | 5.8746183819503 | | 1963 | 6.399369645843 | | 1962 | 5.5405397418821 | | 1961 | 4.17012453699 | | 1960 | 4.5616060181904 |
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