| Year | Value | | 2008 | 2.627582524628 | | 2007 | 2.9010748925344 | | 2006 | 4.5876342404337 | | 2005 | 4.5077567825895 | | 2004 | 5.607179054054 | | 2000 | 5.0691011913243 | | 1999 | 5.4780897130426 | | 1998 | 5.6508745401074 | | 1997 | 5.4749860775942 | | 1996 | 5.0804787934591 | | 1995 | 5.7916460293875 | | 1994 | 5.7574446987837 | | 1993 | 6.1587372413408 | | 1992 | 6.7600280536639 | | 1991 | 6.6508845075731 | | 1983 | 7.6596931659693 | | 1982 | 7.4850640113798 | | 1981 | 1.4153260012204 | | 1980 | 0.53553960174423 | | 1979 | 0.017505470459519 | | 1978 | 0.020942408376963 |
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