| Year | Value | | 2021 | 10.00555140516 | | 2020 | 9.9827532713623 | | 2019 | 9.9517421343192 | | 2018 | 9.8918135906399 | | 2017 | 9.8213373824151 | | 2016 | 9.7363150890993 | | 2015 | 9.6331701408115 | | 2014 | 9.5638339640912 | | 2013 | 9.4748258659544 | | 2012 | 9.312914381337 | | 2011 | 9.0941771866581 | | 2010 | 8.9266387670227 | | 2009 | 8.8248942120324 | | 2008 | 8.584603930463 | | 2007 | 8.4922080772288 | | 2006 | 8.3701345069868 | | 2005 | 8.3109509795871 | | 2004 | 8.1591383954356 | | 2003 | 8.1498797465025 | | 2002 | 7.9362042340398 | | 2001 | 7.8468919071228 | | 2000 | 7.7736970149374 | | 1999 | 7.7749862510408 | | 1998 | 7.6764588611453 | | 1997 | 7.6329719356606 | | 1996 | 7.6185919945083 | | 1995 | 7.5292876013519 | | 1994 | 7.3200842505876 | | 1993 | 7.2596884977479 | | 1992 | 7.1110403224243 | | 1991 | 6.993428330267 | | 1990 | 6.8617128661689 | | 1989 | 6.7882552066605 | | 1988 | 6.7555630596879 | | 1987 | 6.6957105162215 | | 1986 | 6.6865407676804 | | 1985 | 6.653300429219 | | 1984 | 6.5294091527347 | | 1983 | 6.4947734177562 | | 1982 | 6.4517653036749 | | 1981 | 6.4150863095106 | | 1980 | 6.4073617930766 | | 1979 | 6.3845869284936 | | 1978 | 6.3680415126477 | | 1977 | 6.294733359909 | | 1976 | 6.2775899169844 | | 1975 | 6.239964046612 | | 1974 | 6.1869153187114 | | 1973 | 6.2014430715121 | | 1972 | 6.1871408274202 | | 1971 | 6.1560944926842 | | 1970 | 6.1183704551318 | | 1969 | 6.1002808572038 | | 1968 | 6.067938848787 | | 1967 | 5.9930887769194 | | 1966 | 5.962291610199 | | 1965 | 5.9360293013613 | | 1964 | 5.9011956127891 | | 1963 | 5.8536543554245 | | 1962 | 5.7646515402744 | | 1961 | 5.7383892314368 |
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