| Year | Value | | 2009 | 9.1006679774618 | | 2008 | 11.578882810743 | | 2007 | 12.147449513336 | | 2006 | 11.75644070848 | | 2005 | 11.569373913795 | | 2004 | 10.489435834501 | | 2003 | 9.4207018229851 | | 2002 | 9.5079254485281 | | 2001 | 9.8263073889776 | | 2000 | 13.796110041301 | | 1999 | 14.091570465228 | | 1998 | 14.200240474395 | | 1997 | 14.445504438988 | | 1996 | 13.452178834182 | | 1995 | 13.596388492579 | | 1994 | 13.004582151429 | | 1993 | 13.734537322728 | | 1992 | 14.958982364077 | | 1991 | 14.614095298907 | | 1990 | 13.962319263911 | | 1989 | 15.086961596825 | | 1988 | 13.570656579303 | | 1987 | 12.284069097889 | | 1986 | 10.289769367238 | | 1985 | 9.9911386796633 | | 1984 | 9.2616580310881 | | 1983 | 12.631837135148 | | 1982 | 12.219959266802 | | 1981 | 6.8914522330671 | | 1980 | 5.1154086088584 |
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