| Year | Value | | 2023 | 1.0787690601645 | | 2022 | 1.0698458397152 | | 2021 | 1.1636821107288 | | 2020 | 1.3266602686055 | | 2019 | 1.382797246388 | | 2018 | 1.4696777235067 | | 2017 | 1.4140216679759 | | 2016 | 1.3479754129948 | | 2015 | 1.3655171539616 | | 2014 | 1.330244423168 | | 2013 | 1.3294460838256 | | 2012 | 1.3786564647044 | | 2011 | 1.4118511592452 | | 2010 | 1.5394069812836 | | 2009 | 1.5386256760121 | | 2008 | 1.4419240883772 | | 2007 | 1.466292099929 | | 2006 | 1.4810849563629 | | 2005 | 1.5240134627332 | | 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 |
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