| Year | Value | | 2021 | 2.6322706388107 | | 2020 | 2.6561996976187 | | 2019 | 2.6988342822069 | | 2018 | 2.7504618749735 | | 2017 | 2.7776234158393 | | 2016 | 2.8306293197021 | | 2015 | 2.8600779737357 | | 2014 | 2.8833159538207 | | 2013 | 2.9428214108726 | | 2012 | 2.9779648881708 | | 2011 | 3.0174272169593 | | 2010 | 3.06590173914 | | 2009 | 3.1164305219047 | | 2008 | 3.1532511215694 | | 2007 | 3.1946467174486 | | 2006 | 3.2382141908329 | | 2005 | 3.2981662608488 | | 2004 | 3.3641740195708 | | 2003 | 3.4242227794859 | | 2002 | 3.4953603597205 | | 2001 | 3.558506631379 | | 2000 | 3.6056270956648 | | 1999 | 3.6500619657783 | | 1998 | 3.7155506842045 | | 1997 | 3.7940514368807 | | 1996 | 3.8761776414436 | | 1995 | 3.97169510735 | | 1994 | 4.0707236173278 | | 1993 | 4.1708667712687 | | 1992 | 4.2769808572175 | | 1991 | 4.3799642374375 | | 1990 | 4.4775798760226 | | 1989 | 4.5733583248078 | | 1988 | 4.6706886136167 | | 1987 | 4.7689040667959 | | 1986 | 4.8672539162637 | | 1985 | 4.9368452887999 | | 1984 | 5.0274706363449 | | 1983 | 5.0918569207579 | | 1982 | 5.1496897166629 | | 1981 | 5.2115203950787 | | 1980 | 5.2736559970976 | | 1979 | 5.3110055348575 | | 1978 | 5.3603365415682 | | 1977 | 5.4262439144298 | | 1976 | 5.5071638616983 | | 1975 | 5.5569399319195 | | 1974 | 5.6389345388272 | | 1973 | 5.6999737792385 | | 1972 | 5.7569858373528 | | 1971 | 5.8124341887528 | | 1970 | 5.8488965423464 | | 1969 | 5.8841086104941 | | 1968 | 5.9275866447454 | | 1967 | 5.9647815481111 | | 1966 | 6.0004403877593 | | 1965 | 6.0418229383208 | | 1964 | 6.0664667404217 | | 1963 | 6.0699171182928 | | 1962 | 6.0627702565721 | | 1961 | 6.0193215267068 | | 1960 | 6.0340537139356 |
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