| Year | Value | | 2022 | 4.1771295631144 | | 2021 | 2.8953681592645 | | 2020 | 3.2726485882524 | | 2019 | 3.2095622278151 | | 2018 | 3.014504750963 | | 2017 | 3.1254621368489 | | 2016 | 3.622000080858 | | 2015 | 3.4992157872968 | | 2014 | 3.1012818291425 | | 2013 | 2.9792929247148 | | 2012 | 2.9277865603723 | | 2011 | 2.7631367403256 | | 2010 | 2.850167127414 | | 2009 | 3.0861333490525 | | 2008 | 2.6872908026289 | | 2007 | 2.6853757103039 | | 2006 | 2.7487492850157 | | 2005 | 2.7729036339662 | | 2004 | 2.8298068953025 | | 2003 | 3.1951846644661 | | 2002 | 3.4104624333724 | | 2001 | 3.2388455566853 | | 2000 | 3.1982246595999 | | 1999 | 3.2036811621122 | | 1998 | 2.7879476680216 | | 1997 | 3.7038071884928 | | 1996 | 3.5000053931716 | | 1995 | 3.4472627543581 | | 1994 | 3.9330708144975 | | 1993 | 3.6039328404561 | | 1992 | 3.7987402772287 | | 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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