| Year | Value | | 2005 | 17.475447313993 | | 2004 | 19.289801924413 | | 2003 | 11.178098543425 | | 2002 | 6.6771874030547 | | 2001 | 6.7154156418333 | | 2000 | 9.5924534488601 | | 1999 | 12.690771441436 | | 1998 | 15.246676786337 | | 1997 | 17.605665086244 | | 1996 | 24.185543140759 | | 1995 | 18.484881597064 | | 1994 | 10.358022736825 | | 1993 | 17.348475238482 | | 1992 | 12.936286643557 | | 1985 | 5.7204524561407 | | 1984 | 3.9701787913572 | | 1983 | -11.750674386989 | | 1982 | -6.7434974724424 | | 1981 | -2.9424076934638 | | 1980 | 13.311625708588 | | 1979 | 15.83442366015 | | 1978 | 14.121916021568 | | 1977 | 6.6680962414446 |
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