| Year | Value | | 2021 | 9.3534575365615 | | 2020 | 9.3161100480876 | | 2019 | 9.2865054289522 | | 2018 | 9.2530225454324 | | 2017 | 9.1776170052117 | | 2016 | 9.0721604113947 | | 2015 | 8.9584048884754 | | 2014 | 8.902444428432 | | 2013 | 8.8270116596655 | | 2012 | 8.6771850071395 | | 2011 | 8.4045556440023 | | 2010 | 8.2254265022637 | | 2009 | 8.0278721583198 | | 2008 | 7.8263267336325 | | 2007 | 7.7742212972423 | | 2006 | 7.6209833337411 | | 2005 | 7.5381903727021 | | 2004 | 7.3532456270457 | | 2003 | 7.3424893326654 | | 2002 | 7.0137852976173 | | 2001 | 6.9152110912566 | | 2000 | 6.8441411448638 | | 1999 | 6.8268663358195 | | 1998 | 6.8245860610257 | | 1997 | 6.7966699696102 | | 1996 | 6.7433944585177 | | 1995 | 6.6152153754092 | | 1994 | 6.4339680789169 | | 1993 | 6.4269199568268 | | 1992 | 6.2363442634505 | | 1991 | 6.1756060348509 | | 1990 | 6.0916504628958 | | 1989 | 5.974250860631 | | 1988 | 5.969966707988 | | 1987 | 5.9531064943608 | | 1986 | 5.9788805094549 | | 1985 | 5.9679628301389 | | 1984 | 5.9582198378379 | | 1983 | 5.9623657920085 | | 1982 | 5.9473021585219 | | 1981 | 5.9263650899603 | | 1980 | 5.9421197158087 | | 1979 | 5.9272633800306 | | 1978 | 5.8985871970171 | | 1977 | 5.861066311773 | | 1976 | 5.8488357469696 | | 1975 | 5.8010190755351 | | 1974 | 5.8009969362245 | | 1973 | 5.8344860752007 | | 1972 | 5.8384936093649 | | 1971 | 5.7933052068924 | | 1970 | 5.7487386631695 | | 1969 | 5.8004220348047 | | 1968 | 5.7403090223413 | | 1967 | 5.6795741511283 | | 1966 | 5.6599510528069 | | 1965 | 5.6699007928008 | | 1964 | 5.6252651536613 | | 1963 | 5.5640466145319 | | 1962 | 5.5199637387254 | | 1961 | 5.4571560050136 |
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