| Year | Value | | 2007 | 5.1250849010109 | | 2006 | 4.8631160181389 | | 2005 | 5.1919303257751 | | 2004 | 5.0529396573986 | | 2003 | 5.6678672966732 | | 2002 | 6.5330837409839 | | 2001 | 7.338112015551 | | 2000 | 8.1162543728136 | | 1999 | 8.2909329239897 | | 1998 | 8.7768808728007 | | 1997 | 8.9808965674709 | | 1996 | 8.1890480421263 | | 1995 | 11.161455009572 | | 1994 | 10.860168575751 | | 1993 | 10.504382196254 | | 1992 | 11.974275214373 | | 1991 | 11.141859392502 | | 1990 | 13.002237136465 | | 1989 | 14.579169442594 | | 1988 | 17.007753908733 | | 1987 | 19.093213179386 | | 1986 | 18.712108798951 | | 1985 | 15.455475946776 |
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