| Year | Value | | 2014 | 15.021800740114 | | 2013 | 14.756451413437 | | 2012 | 14.359740378293 | | 2011 | 11.860123210801 | | 2010 | 11.446413218023 | | 2009 | 10.360995698704 | | 2008 | 8.4355450729304 | | 2007 | 8.4217083096049 | | 2006 | 7.8580737029779 | | 2005 | 6.0561921425701 | | 2004 | 4.8948384895471 | | 2003 | 3.0895430476379 | | 2002 | 1.5005426389711 | | 2001 | 0.67382887960859 | | 2000 | 0.54840876849748 | | 1999 | 2.2553392199073 | | 1998 | 0.071244173919668 | | 1997 | 0.69549459443134 | | 1996 | -1.6371052295067 | | 1995 | -1.9187571146048 | | 1994 | -1.3699078345256 | | 1993 | 0.4421310490738 | | 1992 | -3.0977563376412 | | 1991 | -4.5658638996774 | | 1990 | -1.1661638515884 | | 1989 | -5.9917975066977 | | 1988 | -2.9684406625307 | | 1987 | -4.3543448099289 | | 1986 | -6.5562122427125 | | 1985 | -7.5437443392684 | | 1984 | -6.237454332955 | | 1983 | -5.1684127346239 | | 1982 | -4.2069423519552 | | 1981 | -3.1717261288929 | | 1980 | -2.9184906736598 | | 1979 | -3.115494120823 | | 1978 | -2.3870136019013 | | 1977 | -2.1244891319489 | | 1976 | -2.3760117117145 | | 1975 | -2.8445261869782 | | 1974 | -1.9035377360829 | | 1973 | -1.11419500722 | | 1972 | 0.81915575017427 | | 1971 | 0.79060731111779 |
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