| Year | Value | | 2015 | 28.546362350418 | | 2014 | 28.391011014674 | | 2013 | 27.332095016479 | | 2012 | 26.976549211496 | | 2011 | 32.518226746529 | | 2010 | 33.210916163462 | | 2009 | 28.601923494678 | | 2008 | 27.345051417298 | | 2007 | 25.044144957876 | | 2006 | 20.103052846641 | | 2005 | 14.965637411775 | | 2004 | 13.74714624992 | | 2003 | 12.399134950583 | | 2002 | 9.7912424096593 | | 2001 | 10.575707205982 | | 2000 | 10.740568861088 | | 1999 | 9.7907991976452 | | 1998 | 8.272008450045 | | 1997 | 2.0958873163044 | | 1996 | 4.3994849399695 | | 1995 | 0.11905638728314 | | 1994 | -1.267770717278 | | 1993 | 3.5647066317218 | | 1992 | 0.90924496839771 | | 1991 | 1.1608473126866 | | 1990 | 0.73686634543613 | | 1989 | 1.2824455183306 | | 1988 | 0.0080138098810441 | | 1987 | 0.64131328133555 | | 1986 | -1.1086284037773 | | 1985 | -3.3191658060678 | | 1984 | -7.7969504312811 | | 1983 | -3.4533383473746 | | 1982 | 0.70954631282414 | | 1981 | 7.1394706781982 | | 1980 | 0.016793553531008 | | 1979 | -7.6481118005506 | | 1978 | -8.0319634419807 | | 1977 | -8.8553227213056 | | 1976 | -10.229279516363 | | 1975 | -12.282899414721 | | 1974 | -14.156035624475 | | 1973 | -15.642292561508 | | 1972 | -16.440086963786 | | 1971 | -15.241312133737 | | 1970 | -14.970166156414 | | 1969 | -15.405856082333 | | 1968 | -16.07235109628 | | 1967 | -15.863751613717 | | 1966 | -17.760415314379 | | 1965 | -17.715073831364 | | 1964 | -19.719407474459 | | 1963 | -17.080467335975 | | 1962 | -14.712290135191 | | 1961 | -17.699936481492 | | 1960 | -19.627350693911 |
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