| Year | Value | | 2022 | 2.0371352161292 | | 2021 | 1.7662746190896 | | 2020 | 1.8295605824169 | | 2019 | 1.6922212928377 | | 2018 | 1.6342413899639 | | 2017 | 1.6484363475512 | | 2016 | 1.6989423124743 | | 2015 | 1.684527026843 | | 2014 | 1.6995836042976 | | 2013 | 1.7308153156423 | | 2012 | 1.7658006225937 | | 2011 | 1.7212931985236 | | 2010 | 1.7688959828751 | | 2009 | 1.8262880397716 | | 2008 | 1.7283564272368 | | 2007 | 1.7035976655537 | | 2006 | 1.7278525756191 | | 2005 | 1.7315041934099 | | 2004 | 1.7935641744972 | | 2003 | 1.8347025379415 | | 2002 | 1.8488645694982 | | 2001 | 1.8271129533615 | | 2000 | 1.8567714351255 | | 1999 | 1.8784741327279 | | 1998 | 1.896473762322 | | 1997 | 2.0083654745103 | | 1996 | 2.015990253482 | | 1995 | 2.0386445801817 | | 1994 | 2.2364362776716 | | 1993 | 2.321338372422 | | 1992 | 2.4232189205989 | | 1991 | 2.4687630455436 | | 1990 | 2.6015681868096 | | 1989 | 2.6699973771179 | | 1988 | 2.7579861819014 | | 1987 | 2.8854953052784 | | 1986 | 2.9247790188982 | | 1985 | 3.066235828217 | | 1984 | 3.1170102737276 | | 1983 | 3.1729857974308 | | 1982 | 3.2055642681678 | | 1981 | 3.1147941514934 | | 1980 | 3.0337474761368 | | 1979 | 3.0282907295799 | | 1978 | 3.0630405462682 | | 1977 | 3.1103291731247 | | 1976 | 3.1316494015856 | | 1975 | 3.2383126753781 | | 1974 | 3.1230249474986 | | 1973 | 3.0969993484685 | | 1972 | 3.2515573149344 | | 1971 | 3.2208527234901 | | 1970 | 3.2334192100263 | | 1969 | 3.4995268620441 | | 1968 | 3.771190209367 | | 1967 | 3.9216006785873 | | 1966 | 3.9850737439528 | | 1965 | 4.0677118672874 | | 1964 | 4.1830237583026 | | 1963 | 4.3005152512533 | | 1962 | 4.4909851819685 | | 1961 | 4.4979288853106 | | 1960 | 4.6177806607827 |
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