| Year | Value | | 2019 | 2.5980162031108 | | 2018 | 4.8150616746151 | | 2017 | 6.2213752893312 | | 2016 | 7.8472745908568 | | 2015 | 4.9512449370492 | | 2014 | 5.6813488577645 | | 2013 | 5.6213062346895 | | 2012 | 8.9396468278821 | | 2011 | 6.1074271459025 | | 2010 | 4.5092369177437 | | 2009 | 7.4482342344575 | | 2008 | 12.657460186391 | | 2007 | 8.0760882786159 | | 2006 | 5.3046394633873 | | 2005 | 4.7740040455371 | | 2004 | 3.4453350731661 | | 2003 | 7.29 | | 2002 | 12.019715469923 | | 2001 | 5.9421098228165 | | 2000 | 12.208535854584 | | 1999 | 6.0888606343149 | | 1998 | 8.1099656357386 | | 1997 | 7.1253272251306 | | 1996 | 6.4252133031522 | | 1995 | 12.288591260143 | | 1994 | 13.769324880436 | | 1993 | 12.023423872415 | | 1992 | 7.5582953631734 | | 1991 | 8.9343065693429 | | 1990 | 13.092289912498 | | 1989 | 7.5461647727277 | | 1988 | 20.394562278112 | | 1987 | 13.376623376623 | | 1986 | 13.737075332349 | | 1985 | 20.462633451958 | | 1984 | 12.939726641273 | | 1983 | 11.567524383407 | | 1982 | 10.807421341841 | | 1981 | 20.055796652254 | | 1980 | 18.683975403496 | | 1979 | 16.453516970076 | | 1978 | 8.5176147629162 | | 1977 | 20.80512182762 | | 1976 | 6.5309126594924 | | 1975 | 12.045741932168 | | 1974 | 19.287775445696 | | 1973 | 11.543525969644 | | 1972 | 2.3663321848999 | | 1971 | 2.321661175556 | | 1970 | 1.8495395657139 | | 1969 | 3.2055412377031 | | 1968 | 3.4494250952424 | | 1967 | 1.8240434379497 | | 1966 | 3.1775210084201 |
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