| Year | Value | | 2022 | 2.7020410893396 | | 2021 | 2.7597051793338 | | 2020 | 2.8291381128221 | | 2019 | 2.8870740849335 | | 2018 | 2.9223921868109 | | 2017 | 2.9463215611881 | | 2016 | 2.9498281439255 | | 2015 | 2.9512491226683 | | 2014 | 2.9262289798469 | | 2013 | 2.8999206091431 | | 2012 | 2.9150121221731 | | 2011 | 2.9281111574055 | | 2010 | 2.9349266629319 | | 2009 | 2.9433536907288 | | 2008 | 2.9477423133012 | | 2007 | 2.8755572413652 | | 2006 | 3.059823472555 | | 2005 | 3.177348170646 | | 2004 | 3.0264570510548 | | 2003 | 3.0142781128238 | | 2002 | 3.0013812728426 | | 2001 | 3.0124309700982 | | 2000 | 3.038456621386 | | 1999 | 3.0572856737354 | | 1998 | 3.0401512302295 | | 1997 | 2.9097066366637 | | 1996 | 2.5690267497473 | | 1995 | 2.0571816048938 | | 1994 | 3.7359622143217 | | 1993 | 4.4508226064372 | | 1992 | 3.0647863657596 | | 1991 | 3.062197463777 | | 1990 | 3.0406276980036 | | 1989 | 3.0182988908611 | | 1988 | 3.0008729423437 | | 1987 | 2.9742990503755 | | 1986 | 2.9392589621546 | | 1985 | 2.903623876325 | | 1984 | 2.8720178612723 | | 1983 | 2.848290465701 | | 1982 | 2.8220144063323 | | 1981 | 2.7686133169813 | | 1980 | 2.6842883388715 | | 1979 | 2.5910756515767 | | 1978 | 2.5190863274371 | | 1977 | 2.4584559787176 | | 1976 | 2.3871763212461 | | 1975 | 2.318284763911 | | 1974 | 2.2638397835609 | | 1973 | 2.2235990372658 | | 1972 | 2.1787666813786 | | 1971 | 2.12507571508 | | 1970 | 2.0813138540757 | | 1969 | 2.0452460872083 | | 1968 | 2.0010913239342 | | 1967 | 1.9564996591318 | | 1966 | 1.9115123913557 | | 1965 | 1.8528373645098 | | 1964 | 1.7859103461118 | | 1963 | 1.7120170779695 | | 1962 | 1.6361858934462 | | 1961 | 1.5377808712231 |
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