| Year | Value | | 2015 | 35.021007653641 | | 2014 | 19.769289350984 | | 2013 | 10.384686264116 | | 2012 | 16.852487731514 | | 2011 | 16.775616864361 | | 2010 | 16.302849203475 | | 2009 | 19.197105955943 | | 2008 | 16.412336322563 | | 2007 | 23.722802998142 | | 2006 | 23.730695397638 | | 2005 | 23.185788263217 | | 2004 | 17.594024688679 | | 2003 | 27.510646601671 | | 2002 | 22.626859584511 | | 2001 | 20.502915880362 | | 2000 | 23.279911706659 | | 1999 | 19.962590184067 | | 1998 | 15.951874803473 | | 1997 | 11.881117896861 | | 1996 | 3.9738898524001 | | 1995 | 9.4676191789798 | | 1994 | 4.2620338987789 | | 1993 | 0.57273570306288 | | 1992 | 1.4088618017416 | | 1991 | 2.8600928121635 | | 1990 | 7.8245839284171 | | 1989 | 7.2851420789873 | | 1988 | 13.346249638405 | | 1987 | 3.7315903760758 | | 1986 | 0.94767668742179 | | 1985 | -9.3922518821442 | | 1984 | -5.1992098192194 | | 1983 | -9.2821716169479 | | 1982 | -6.0547659167459 | | 1981 | -9.7453573055134 | | 1980 | -11.584716100426 | | 1979 | -7.6031006439896 | | 1978 | -7.6014322915144 | | 1977 | -20.336656391085 | | 1976 | -16.81582120167 | | 1975 | -20.859761423885 | | 1974 | -9.4113077548548 | | 1973 | 8.4474052979908 | | 1972 | 18.173511630813 | | 1971 | 26.589807009066 | | 1970 | 41.199833848798 | | 1969 | 48.302035841751 | | 1968 | 55.595511430001 | | 1967 | 63.204170182669 | | 1966 | 64.779340363985 | | 1965 | 63.835280978397 | | 1964 | 63.628040083941 | | 1963 | 62.879315453609 | | 1962 | 59.391004704699 | | 1961 | 52.004779138369 | | 1960 | 51.368302589231 |
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