| Year | Value | | 2022 | 2.2271839786605 | | 2021 | 2.1615719177569 | | 2020 | 2.1567775600424 | | 2019 | 1.9813273003125 | | 2018 | 1.9399534627169 | | 2017 | 1.9476663878508 | | 2016 | 1.9767334868622 | | 2015 | 2.0440582412761 | | 2014 | 2.1841184454461 | | 2013 | 2.2911179130038 | | 2012 | 2.4184864177748 | | 2011 | 2.4966057256291 | | 2010 | 2.567999604672 | | 2009 | 2.647724532598 | | 2008 | 2.4871500848402 | | 2007 | 2.3746577712774 | | 2006 | 2.3696744028424 | | 2005 | 2.4227160757339 | | 2004 | 2.4978213675163 | | 2003 | 2.5520620151824 | | 2002 | 2.485659674707 | | 2001 | 2.4111221013072 | | 2000 | 2.3727966452319 | | 1999 | 2.4229095933237 | | 1998 | 2.4959622750188 | | 1997 | 2.5601487722561 | | 1996 | 2.7245923219832 | | 1995 | 2.8541994905359 | | 1994 | 3.3819337177653 | | 1993 | 3.5906495124798 | | 1992 | 3.8647177664126 | | 1991 | 4.1168024227317 | | 1990 | 3.983217694585 | | 1989 | 4.0409543315643 | | 1988 | 4.2142876283885 | | 1987 | 4.6866684426053 | | 1986 | 5.0598237367973 | | 1985 | 5.3207046190555 | | 1984 | 5.5080794980595 | | 1983 | 5.3968855900151 | | 1982 | 5.3813265211548 | | 1981 | 4.9837633692175 | | 1980 | 5.0199034588874 | | 1979 | 4.6797575674753 | | 1978 | 4.7330430858699 | | 1977 | 4.9232776210389 | | 1976 | 5.1755033368155 | | 1975 | 5.3396034391561 | | 1974 | 5.0694271149054 | | 1973 | 4.9061610574651 | | 1972 | 5.1710477559332 | | 1971 | 5.076330622546 | | 1970 | 5.1974132380957 | | 1969 | 5.5021956389853 | | 1968 | 5.9411052003122 | | 1967 | 6.2357037120617 | | 1966 | 6.2490367967667 | | 1965 | 6.4369785734014 | | 1964 | 6.5960560131352 | | 1963 | 6.8007434106824 | | 1962 | 6.944166041059 | | 1961 | 6.9213875228592 | | 1960 | 7.0924863708221 |
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