| Year | Value | | 2021 | 2.2409300246198 | | 2020 | 2.2670977826263 | | 2019 | 2.3201159971577 | | 2018 | 2.3793410111731 | | 2017 | 2.4061305272959 | | 2016 | 2.4751853554712 | | 2015 | 2.4983659247318 | | 2014 | 2.5300084279949 | | 2013 | 2.6213319058973 | | 2012 | 2.6783483755127 | | 2011 | 2.7427880984949 | | 2010 | 2.80760182405 | | 2009 | 2.8777458992535 | | 2008 | 2.9296329500551 | | 2007 | 2.9995918654233 | | 2006 | 3.0737458508464 | | 2005 | 3.1669544021735 | | 2004 | 3.2580464510328 | | 2003 | 3.3392051761402 | | 2002 | 3.4322681674566 | | 2001 | 3.5155477558257 | | 2000 | 3.5722240762342 | | 1999 | 3.6187098388723 | | 1998 | 3.6863160178773 | | 1997 | 3.7619424619181 | | 1996 | 3.8300644474001 | | 1995 | 3.9059069934391 | | 1994 | 3.9881144341873 | | 1993 | 4.0627444414799 | | 1992 | 4.1457441556914 | | 1991 | 4.236102406265 | | 1990 | 4.3318882973183 | | 1989 | 4.4234316318511 | | 1988 | 4.5126305303452 | | 1987 | 4.6070250374572 | | 1986 | 4.7002094752098 | | 1985 | 4.7520109781255 | | 1984 | 4.8377699067838 | | 1983 | 4.893988663833 | | 1982 | 4.9617063722406 | | 1981 | 5.0406875774186 | | 1980 | 5.1121330889667 | | 1979 | 5.1504095486454 | | 1978 | 5.2212209324079 | | 1977 | 5.3197551714745 | | 1976 | 5.4233640554528 | | 1975 | 5.4790953172633 | | 1974 | 5.5854073446239 | | 1973 | 5.6493487940183 | | 1972 | 5.717197290693 | | 1971 | 5.7867595882641 | | 1970 | 5.8372965037334 | | 1969 | 5.8856290078727 | | 1968 | 5.9476920622036 | | 1967 | 6.0015787038622 | | 1966 | 6.0431765595411 | | 1965 | 6.093012453991 | | 1964 | 6.1263229333878 | | 1963 | 6.1199292384074 | | 1962 | 6.1098587827407 | | 1961 | 6.0944659652717 | | 1960 | 6.078442883051 |
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