| Year | Value | | 2022 | 4.9682856258069 | | 2021 | 5.0969575655805 | | 2020 | 5.7711366181681 | | 2019 | 5.4064745285131 | | 2018 | 5.9003267949722 | | 2017 | 6.2793096922368 | | 2016 | 6.0458049874266 | | 2015 | 7.3937864966055 | | 2014 | 6.3728960606175 | | 2013 | 5.7331965378347 | | 2012 | 4.9711601977073 | | 2011 | 4.8860158645612 | | 2010 | 4.9021026427116 | | 2009 | 5.1261379916341 | | 2008 | 4.0918941160905 | | 2007 | 4.4244901965527 | | 2006 | 4.3176908998842 | | 2005 | 4.5991624873875 | | 2004 | 5.0758813958704 | | 2003 | 5.4749118925666 | | 2002 | 5.7895806218603 | | 2001 | 6.2681138302359 | | 2000 | 6.2827134341652 | | 1999 | 6.4015671056976 | | 1998 | 7.3233927339098 | | 1997 | 6.5120661481208 | | 1996 | 6.0145009054417 | | 1995 | 6.6209027738406 | | 1994 | 7.3909430694245 | | 1993 | 7.8047298927048 | | 1992 | 8.6160340822058 | | 1991 | 10.747746890007 | | 1989 | 7.6968701115907 | | 1988 | 8.0608258046226 | | 1987 | 8.7472586968605 | | 1985 | 10.494745331173 | | 1984 | 11.586973662889 | | 1983 | 11.071144468747 | | 1982 | 12.099058315669 | | 1981 | 9.9221167056186 | | 1980 | 8.5640005600398 | | 1979 | 9.669486786121 | | 1978 | 10.575469617276 | | 1977 | 10.863228081117 | | 1973 | 8.0212420769444 | | 1972 | 7.5442801338595 | | 1971 | 8.0658363361118 | | 1970 | 8.2711040761116 | | 1969 | 8.1148228940669 | | 1968 | 7.3572281008315 | | 1967 | 5.9955548762219 | | 1966 | 5.9005525892456 | | 1965 | 6.0789525999695 | | 1964 | 4.7924809361169 | | 1963 | 3.8814296085219 | | 1962 | 5.3547258588071 | | 1961 | 4.7198689458744 |
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