| Year | Value | | 2007 | 16.964865737926 | | 2006 | 16.816168162387 | | 2005 | 16.21516410276 | | 2004 | 15.016348801467 | | 2003 | 12.973000318333 | | 2002 | 13.184104976033 | | 2001 | 11.782546046114 | | 2000 | 12.330802881141 | | 1999 | 12.039785186592 | | 1998 | 13.546529150729 | | 1997 | 12.620346024982 | | 1996 | 12.293852523875 | | 1995 | 9.7223294940959 | | 1994 | 10.045468427209 | | 1993 | 9.5163454005398 | | 1992 | 8.8997886163927 | | 1991 | 7.9327295750991 | | 1990 | 7.2362245932934 | | 1989 | 7.0947432047591 | | 1988 | 7.2805514762364 | | 1987 | 7.949052567139 | | 1986 | 8.0199545289869 | | 1985 | 8.2581140924121 |
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