| Year | Value | | 2018 | 1.4904448678068 | | 2017 | 1.5910592088242 | | 2016 | 1.6322506412044 | | 2015 | 1.6660516818806 | | 2014 | 1.7019723862963 | | 2013 | 1.6942025845888 | | 2012 | 1.6329579353093 | | 2011 | 1.5324008169276 | | 2010 | 1.4000928903522 | | 2009 | 1.2738919001843 | | 2008 | 1.195376088951 | | 2007 | 1.1927942540682 | | 2006 | 1.2488663088952 | | 2005 | 1.323138268324 | | 2004 | 1.3847339240455 | | 2003 | 1.4387212948271 | | 2002 | 1.4738810374891 | | 2001 | 1.49422978355 | | 2000 | 1.518097003141 | | 1999 | 1.5495273255711 | | 1998 | 1.5729007140215 | | 1997 | 1.5848727150819 | | 1996 | 1.5890896372015 | | 1995 | 1.589428988184 | | 1994 | 1.5941781802111 | | 1993 | 1.6100594174098 | | 1992 | 1.6427927000404 | | 1991 | 1.6896506185725 | | 1990 | 1.7383958233911 | | 1989 | 1.7873022048064 | | 1988 | 1.8474547500968 | | 1987 | 1.9212052714831 | | 1986 | 2.0040549215683 | | 1985 | 2.097356074412 | | 1984 | 2.1866801389181 | | 1983 | 2.2515958165487 | | 1982 | 2.2788667055236 | | 1981 | 2.2761618845888 | | 1980 | 2.2581846412608 | | 1979 | 2.244016898115 | | 1978 | 2.2411381105977 | | 1977 | 2.2602540029062 | | 1976 | 2.2961760046372 |
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