| Year | Value | | 2022 | 2.7441930613512 | | 2021 | 2.8421123489546 | | 2020 | 2.9048196593095 | | 2019 | 2.8865665351477 | | 2018 | 2.9468871505253 | | 2017 | 3.1031851393148 | | 2016 | 3.1291554991688 | | 2015 | 3.0430197171563 | | 2014 | 3.0689338689503 | | 2013 | 3.0727391641718 | | 2012 | 3.0146102992661 | | 2011 | 2.9326775613595 | | 2010 | 2.7998209357293 | | 2009 | 2.66980274555 | | 2008 | 2.620642944057 | | 2007 | 2.5921319211796 | | 2006 | 2.5637800990406 | | 2005 | 2.5897761215382 | | 2004 | 2.6114744795382 | | 2003 | 2.596692516071 | | 2002 | 2.5694031417063 | | 2001 | 2.5131680464721 | | 2000 | 2.453305505825 | | 1999 | 2.4209286872806 | | 1998 | 2.4009045137822 | | 1997 | 2.7127656144417 | | 1996 | 3.0609015786406 | | 1995 | 4.4714649385996 | | 1994 | 4.8542509499681 | | 1993 | 2.7791184726152 | | 1992 | 1.8680174783143 | | 1991 | 1.9186854073604 | | 1990 | 1.6358553923058 | | 1989 | 1.0046936852975 | | 1988 | 0.061886697473711 | | 1987 | 0.42509723105405 | | 1986 | 1.7955405311246 | | 1985 | 2.0222077456685 | | 1984 | 2.1383474418366 | | 1983 | 2.1733605559041 | | 1982 | 2.2243186782559 | | 1981 | 1.9644243136516 | | 1980 | 2.5354051994449 | | 1979 | 3.5370324332658 | | 1978 | 3.5782856972669 | | 1977 | 3.3793004292833 | | 1976 | 3.199369645008 | | 1975 | 3.1351922743337 | | 1974 | 3.0286714828936 | | 1973 | 2.9259126685445 | | 1972 | 2.7192494217748 | | 1971 | 2.4799554614043 | | 1970 | 2.3302388156165 | | 1969 | 2.2445518551773 | | 1968 | 2.2153532608406 | | 1967 | 2.1844942864448 | | 1966 | 2.1755979447173 | | 1965 | 2.1564139166882 | | 1964 | 2.1112057337371 | | 1963 | 2.0943874683951 | | 1962 | 2.0440839424215 | | 1961 | 1.8845878057897 |
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