| Year | Value | | 2018 | 1.4749015473959 | | 2017 | 1.4195266380702 | | 2016 | 1.350150572887 | | 2015 | 1.3655171539616 | | 2014 | 1.330244423168 | | 2013 | 1.3294461087607 | | 2012 | 1.3786564647044 | | 2011 | 1.4118511592452 | | 2010 | 1.5394069812836 | | 2009 | 1.5386256760121 | | 2008 | 1.4419240420101 | | 2007 | 1.466292099929 | | 2006 | 1.481084894893 | | 2005 | 1.5240133925211 | | 2004 | 1.4612684089062 | | 2003 | 1.5034776355451 | | 2002 | 1.8957711991562 | | 2001 | 1.9518824453253 | | 2000 | 1.7307261471363 | | 1999 | 1.6454746665699 | | 1998 | 1.6622919197582 | | 1997 | 1.5776883155177 | | 1996 | 1.6548434995029 | | 1995 | 1.8621369803338 | | 1994 | 2.016007789121 | | 1993 | 1.927922814983 | | 1992 | 1.5215288611544 | | 1991 | 1.9633333333333 | | 1990 | 2.363841025197 | | 1989 | 2.68625 | | 1988 | 2.1180722049862 | | 1987 | 1.7439940314537 | | 1986 | 1.2203887275014 | | 1985 | 1.4295231221895 | | 1984 | 1.1995410451653 | | 1983 | 1.2545803230047 | | 1982 | 1.303534361075 | | 1981 | 1.1431487681888 | | 1980 | 1.1384344207436 | | 1979 | 1.5219316860733 | | 1978 | 1.5996715503281 | | 1977 | 1.6877264780016 | | 1976 | 2.0027533651306 | | 1975 | 1.972143796259 | | 1974 | 1.9413534112089 | | 1973 | 2.1546513064924 | | 1972 | 2.435811990225 | | 1971 | 2.619046858581 | | 1970 | 2.8872895569284 | | 1969 | 2.6155900062665 | | 1968 | 2.6497401583638 | | 1967 | 2.8569720083184 | | 1966 | 3.0219816316073 | | 1965 | 3.352092156749 | | 1964 | 2.0144980909483 | | 1963 | 2.2608391823983 | | 1962 | 2.3986833073784 | | 1961 | 2.3640661938534 | | 1960 | 2.6791347259072 |
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