| Year | Value | | 2022 | 3.7594710994912 | | 2021 | 2.7142043037355 | | 2020 | 3.000379520062 | | 2019 | 2.9186938436573 | | 2018 | 2.7770481822595 | | 2017 | 2.8583227567458 | | 2016 | 3.235783108154 | | 2015 | 3.1676868550814 | | 2014 | 2.8659940573439 | | 2013 | 2.7686789680774 | | 2012 | 2.7214491064869 | | 2011 | 2.5656625359008 | | 2010 | 2.6210253864075 | | 2009 | 2.7717495029559 | | 2008 | 2.4723580671875 | | 2007 | 2.5049574591964 | | 2006 | 2.5641309060473 | | 2005 | 2.5835797869029 | | 2004 | 2.6109787423444 | | 2003 | 2.8864927512297 | | 2002 | 3.0451159904528 | | 2001 | 2.9048402135166 | | 2000 | 2.897794681722 | | 1999 | 2.9424501284948 | | 1998 | 2.7311418723174 | | 1997 | 3.4999479432334 | | 1996 | 3.3153500462047 | | 1995 | 3.3211068285721 | | 1994 | 3.6680373597605 | | 1993 | 3.4510709584957 | | 1992 | 3.6363595553238 | | 1986 | 3.691610484742 | | 1985 | 3.5892304005493 | | 1984 | 3.6728484737498 | | 1983 | 3.9376052054601 | | 1982 | 4.2981513121269 | | 1981 | 3.8245318699791 | | 1980 | 3.9003132966797 | | 1979 | 3.3572585580073 | | 1978 | 4.1873064036918 | | 1977 | 4.7136230237622 | | 1976 | 4.9430272108843 | | 1975 | 5.1194539249147 | | 1974 | 3.1930213795885 | | 1973 | 3.3494505494505 | | 1972 | 3.4455205811138 | | 1971 | 3.499793814433 | | 1970 | 3.2564885496183 | | 1969 | 3.0793378995434 | | 1968 | 3.2755555555556 | | 1967 | 3.2595744680851 | | 1966 | 3.1439811172305 | | 1965 | 3.5544186046512 | | 1964 | 3.4224652087475 | | 1963 | 3.387339055794 | | 1962 | 3.7110834371108 | | 1961 | 3.7645429362881 | | 1960 | 3.5315712187959 |
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