| Year | Value | | 2017 | 5.5015799591205 | | 2016 | 5.4544373559331 | | 2015 | 5.4818645636675 | | 2014 | 5.6808510383293 | | 2013 | 5.7341726851278 | | 2012 | 5.8012877001791 | | 2011 | 5.793552662888 | | 2010 | 5.8678991197811 | | 2009 | 5.9179883212916 | | 2008 | 5.942645782167 | | 2007 | 6.0237873722544 | | 2006 | 6.019865101852 | | 2005 | 6.0672135866447 | | 2004 | 6.1597102902924 | | 2003 | 6.2135563515715 | | 2002 | 6.3418226667475 | | 2001 | 6.4091756084511 | | 2000 | 6.5159251674237 | | 1999 | 6.7782268267094 | | 1998 | 6.9206072491705 | | 1997 | 7.0876239260059 | | 1996 | 7.2808208168266 | | 1995 | 7.4889570537038 | | 1994 | 7.7421156126229 | | 1993 | 7.7621405700305 | | 1992 | 6.7389824599125 | | 1991 | 7.0384405589912 | | 1990 | 7.0234634309695 | | 1985 | 8.6837599953642 | | 1980 | 8.9708756762523 | | 1970 | 9.4367926185217 | | 1960 | 9.2860763881571 |
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