| Year | Value | | 2020 | 8.2768581143722 | | 2019 | 2.0747643648503 | | 2018 | 3.7029183911151 | | 2017 | 2.856795755685 | | 2016 | -1.0154819169764 | | 2015 | 4.0419441072562 | | 2014 | 1.3858345458126 | | 2013 | 4.338938483291 | | 2012 | 7.1103706232641 | | 2011 | 2.5592677223094 | | 2010 | -2.4046387510823 | | 2009 | 31.754440774418 | | 2008 | 36.964758287529 | | 2007 | 5.3206002728514 | | 2006 | 0.35346549869376 | | 2005 | 0.90718771807394 | | 2004 | 3.857303913674 | | 2003 | 3.3025538640709 | | 2002 | 0.17499999999992 | | 2001 | 5.9696220416816 | | 2000 | 6.2687687687688 | | 1999 | 6.3473053892215 | | 1998 | 2.5798525798524 | | 1997 | 0.61804697156992 | | 1996 | -1.1002444987775 | | 1995 | 0.24390243902448 | | 1994 | 1.7369727047147 | | 1993 | 1.3836477987422 | | 1992 | 3.2467532467532 | | 1991 | 1.9867549668873 | | 1990 | 3.8888888888891 | | 1989 | 1.5801354401806 | | 1988 | 1.8390804597701 | | 1987 | 2.5943396226414 | | 1986 | 0.23640661938538 | | 1985 | 0.83432657926101 | | 1984 | 4.0942928039701 | | 1983 | 6.0526315789474 | | 1982 | 0.91264667535856 | | 1981 | 10.576790396092 | | 1980 | 13.571540510809 | | 1979 | 12.480169222314 | | 1978 | 11.783251231892 | | 1977 | 14.959168522751 | | 1976 | 14.868733630048 | | 1975 | 18.611372010313 | | 1974 | 24.424667385921 | | 1973 | 18.239795918822 | | 1972 | 20.940994987157 | | 1971 | 14.599292869816 |
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