| Year | Value | | 2021 | 30.264630874478 | | 2020 | 30.560531145806 | | 2019 | 30.870895818243 | | 2018 | 31.084938699539 | | 2017 | 31.321344650083 | | 2016 | 31.700145798859 | | 2015 | 32.030884870574 | | 2014 | 32.342634954344 | | 2013 | 32.701625493946 | | 2012 | 33.033612958595 | | 2011 | 33.430995120321 | | 2010 | 33.852069158018 | | 2009 | 34.23780949581 | | 2008 | 34.619650359312 | | 2007 | 35.047627506033 | | 2006 | 35.388525461565 | | 2005 | 35.71744569018 | | 2004 | 36.111786453403 | | 2003 | 36.495314393468 | | 2002 | 36.887323773385 | | 2001 | 37.224408149948 | | 2000 | 37.525619768554 | | 1999 | 37.857474192442 | | 1998 | 38.135993414261 | | 1997 | 38.567048756099 | | 1996 | 38.904536349028 | | 1995 | 39.352769678429 | | 1994 | 39.671476735218 | | 1993 | 40.063214439113 | | 1992 | 40.441827597982 | | 1991 | 40.914263651033 | | 1990 | 41.4332805557 | | 1989 | 41.902168099168 | | 1988 | 42.28292648649 | | 1987 | 42.839666352853 | | 1986 | 43.30360459649 | | 1985 | 43.803589984455 | | 1984 | 44.190050951899 | | 1983 | 44.472851120477 | | 1982 | 44.805847619951 | | 1981 | 45.03596937277 | | 1980 | 45.22973285427 | | 1979 | 45.298049283673 | | 1978 | 45.190197355765 | | 1977 | 45.226923374842 | | 1976 | 45.33048588726 | | 1975 | 45.428478290338 | | 1974 | 45.629641002211 | | 1973 | 45.874635284044 | | 1972 | 46.202338967597 | | 1971 | 46.340689220267 | | 1970 | 46.374078177041 | | 1969 | 46.442416722104 | | 1968 | 46.512937777053 | | 1967 | 46.626396284054 | | 1966 | 46.6745294734 | | 1965 | 46.838609557538 | | 1964 | 47.021042899777 | | 1963 | 47.179479578207 | | 1962 | 47.3548710539 | | 1961 | 47.425554960143 | | 1960 | 47.486501144984 |
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