| Year | Value | | 2021 | 2.0440137472201 | | 2020 | 2.043276333685 | | 2019 | 2.0398933709011 | | 2018 | 2.0416524327033 | | 2017 | 2.027140474024 | | 2016 | 2.0053911287802 | | 2015 | 1.9380693360005 | | 2014 | 2.0149327930791 | | 2013 | 1.9338059953095 | | 2012 | 2.0292612062492 | | 2011 | 2.0686033982839 | | 2010 | 2.0510513615995 | | 2009 | 2.0426227829052 | | 2008 | 2.0029319967814 | | 2007 | 1.934585716762 | | 2006 | 1.9275532605366 | | 2005 | 1.892959050737 | | 2004 | 1.8445849680104 | | 2003 | 1.8259133355887 | | 2002 | 1.8790211012932 | | 2001 | 1.9039804723508 | | 2000 | 2.0493580354662 | | 1999 | 1.9853077026333 | | 1998 | 1.9823357592647 | | 1997 | 2.018494403583 | | 1996 | 2.030332644668 | | 1995 | 2.024933614215 | | 1994 | 2.0473717866481 | | 1993 | 2.039042276548 | | 1992 | 1.9837062223196 | | 1991 | 1.4985423975267 | | 1990 | 1.546641766588 | | 1989 | 1.5243567339621 | | 1988 | 1.5404906942342 | | 1987 | 1.5253651064791 | | 1986 | 1.4990465837852 | | 1985 | 1.522037477173 | | 1984 | 1.5271801770097 | | 1983 | 1.505500167894 | | 1982 | 1.50348342286 | | 1981 | 1.4850302057987 | | 1980 | 1.4919879761661 | | 1979 | 1.4989457465335 | | 1978 | 1.4923913251729 | | 1977 | 1.4601234046286 | | 1976 | 1.4878536488464 | | 1975 | 1.4830134607647 | | 1974 | 1.5145755205471 | | 1973 | 1.5453308823159 | | 1972 | 1.5337345983703 | | 1971 | 1.5170964518396 | | 1970 | 1.5150797068056 | | 1969 | 1.5009624915675 | | 1968 | 1.4979373740165 | | 1967 | 1.4893662076219 | | 1966 | 1.4727280610912 | | 1965 | 1.4626443359212 | | 1964 | 1.4535689832681 | | 1963 | 1.4460061893905 | | 1962 | 1.4449978168735 | | 1961 | 1.4389475817715 |
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