| Year | Value | | 2021 | 2.2731633524576 | | 2020 | 2.299666175745 | | 2019 | 2.3606087181835 | | 2018 | 2.3974074248687 | | 2017 | 2.4592722382143 | | 2016 | 2.4811555997312 | | 2015 | 2.4779095720192 | | 2014 | 2.5065784810457 | | 2013 | 2.5142484261976 | | 2012 | 2.5496430991073 | | 2011 | 2.5340600663099 | | 2010 | 2.5545817387479 | | 2009 | 2.5797615054047 | | 2008 | 2.5935999078224 | | 2007 | 2.5942702849161 | | 2006 | 2.5952791001781 | | 2005 | 2.6031561062806 | | 2004 | 2.6213987900812 | | 2003 | 2.6300372487444 | | 2002 | 2.6537468388285 | | 2001 | 2.6806478691071 | | 2000 | 2.7184649686098 | | 1999 | 2.7090891880037 | | 1998 | 2.7360586063014 | | 1997 | 2.7713741339653 | | 1996 | 2.8135516710789 | | 1995 | 2.8614373833261 | | 1994 | 2.9155059569439 | | 1993 | 2.9693726091761 | | 1992 | 3.0380369287578 | | 1991 | 3.1244117049421 | | 1990 | 3.3054414910376 | | 1989 | 3.3463430368734 | | 1988 | 3.3961312378874 | | 1987 | 3.4876208531063 | | 1986 | 3.5173197069265 | | 1985 | 3.531215354862 | | 1984 | 3.568344445908 | | 1983 | 3.5943379150599 | | 1982 | 3.7230454793065 | | 1981 | 3.7080683263415 | | 1980 | 3.7347134011225 | | 1979 | 3.7609726869445 | | 1978 | 3.7783014509481 | | 1977 | 3.8463965388514 | | 1976 | 3.9723146544207 | | 1975 | 4.0798998170592 | | 1974 | 4.2637187134469 | | 1973 | 4.4244596596669 | | 1972 | 4.5543838702676 | | 1971 | 4.6959192603581 | | 1970 | 4.8463105614841 | | 1969 | 4.8862482124134 | | 1968 | 4.9824336129872 | | 1967 | 4.8723089638486 | | 1966 | 4.9774561023685 | | 1965 | 5.0935406070457 | | 1964 | 5.1414204443916 | | 1963 | 5.3234282967304 | | 1962 | 5.0351731925265 | | 1961 | 4.5755868233123 | | 1960 | 4.6958544774726 |
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