| Year | Value | | 2021 | 8.7246211382523 | | 2020 | 8.0335290944802 | | 2019 | 7.4716520724394 | | 2018 | 7.4890869443572 | | 2017 | 7.4955728635239 | | 2016 | 7.5135192913171 | | 2015 | 7.5715591868236 | | 2014 | 7.5815970228971 | | 2013 | 7.6591312429927 | | 2012 | 7.7319035068456 | | 2011 | 7.7858706018574 | | 2010 | 7.8747853704737 | | 2009 | 7.9186531856443 | | 2008 | 8.0356866903614 | | 2007 | 8.038437278408 | | 2006 | 8.0739740695516 | | 2005 | 8.1538653840517 | | 2004 | 8.2029270248673 | | 2003 | 8.2991229803867 | | 2002 | 8.3620082526278 | | 2001 | 8.4187813408978 | | 2000 | 8.4935952078075 | | 1999 | 8.5853053892048 | | 1998 | 8.6981780900569 | | 1997 | 8.7561647583135 | | 1996 | 8.8870629412424 | | 1995 | 8.9861227536507 | | 1994 | 9.1059904130805 | | 1993 | 9.1428443118947 | | 1992 | 9.1400970862632 | | 1991 | 9.2238354895598 | | 1990 | 9.2614486016219 | | 1989 | 9.3009973641754 | | 1988 | 9.5291196762128 | | 1987 | 9.5638771326461 | | 1986 | 9.7338363996493 | | 1985 | 9.9225597516058 | | 1984 | 10.043421679674 | | 1983 | 10.166316626217 | | 1982 | 10.117203055254 | | 1981 | 10.198792902834 | | 1980 | 10.350250627312 | | 1979 | 10.38272998293 | | 1978 | 10.549109573771 | | 1977 | 10.787706276766 | | 1976 | 11.150628593752 | | 1975 | 11.307237450351 | | 1974 | 11.383593147014 | | 1973 | 11.468906179853 | | 1972 | 11.761788503557 | | 1971 | 12.263260205223 | | 1970 | 12.139912792878 | | 1969 | 12.287532690333 | | 1968 | 12.440711542187 | | 1967 | 12.687248669949 | | 1966 | 12.939888635208 | | 1965 | 13.357330969516 | | 1964 | 13.529319946449 | | 1963 | 13.459175035953 | | 1962 | 13.616548215112 | | 1961 | 14.583360181758 | | 1960 | 17.234219486472 |
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