| Year | Value | | 2021 | 1.7671061904531 | | 2020 | 1.7630808088959 | | 2019 | 1.7577104953116 | | 2018 | 1.7536851137544 | | 2017 | 1.748316145102 | | 2016 | 1.7429458315177 | | 2015 | 1.7375755179333 | | 2014 | 1.7335501363761 | | 2013 | 1.7281798227918 | | 2012 | 1.7228095092074 | | 2011 | 1.717439195623 | | 2010 | 1.7120702269707 | | 2009 | 1.7080448454135 | | 2008 | 1.7026745318292 | | 2007 | 1.6973042182448 | | 2006 | 1.8156582366327 | | 2005 | 1.9501514393463 | | 2004 | 2.017398040703 | | 2003 | 2.017398040703 | | 2002 | 2.2191378447733 | | 2001 | 2.3536310474869 | | 2000 | 2.3536310474869 | | 1999 | 2.4208776488436 | | 1998 | 2.6616204817008 | | 1997 | 2.6656552777823 | | 1996 | 2.6696900738637 | | 1995 | 2.8512558975269 | | 1994 | 3.0610652937601 | | 1993 | 3.0987233905198 | | 1992 | 3.2816341462103 | | 1991 | 3.6111424928584 | | 1990 | 3.7684995400332 | | 1989 | 4.1289413233055 | | 1988 | 4.114147071007 | | 1987 | 4.1370109154683 | | 1986 | 4.510902019012 | | 1985 | 4.6319459014541 | | 1984 | 4.6736387942953 | | 1983 | 4.7449201917335 | | 1982 | 4.8148566571446 | | 1981 | 5.0206312572963 | | 1980 | 5.1591592560912 | | 1979 | 5.2304406535294 | | 1978 | 5.3689686523243 | | 1977 | 5.5074966511192 | | 1976 | 5.6460246499142 | | 1975 | 5.584157776666 | | 1974 | 5.584157776666 | | 1973 | 5.5169111753092 | | 1972 | 5.3824179725957 | | 1971 | 5.3151713712389 | | 1970 | 5.2479247698821 | | 1969 | 5.1107417031143 | | 1968 | 5.0838430625716 | | 1967 | 5.0569444220289 | | 1966 | 5.0300457814862 | | 1965 | 5.0031471409435 | | 1964 | 4.9762485004008 | | 1963 | 4.9493498598581 | | 1962 | 4.9224512193154 | | 1961 | 4.8955525787727 |
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