| Year | Value | | 2014 | 10.728060251596 | | 2013 | 11.287573408441 | | 2012 | 11.215326277014 | | 2011 | 9.1243050394262 | | 2010 | 7.5227633792355 | | 2009 | 6.5534287669094 | | 2008 | 7.3709773907412 | | 2007 | 7.3851758516163 | | 2006 | 8.0940559460328 | | 2005 | 8.0430294979631 | | 2004 | 8.362155897211 | | 2003 | 6.798665810489 | | 2002 | 6.6259778546306 | | 2001 | 6.0195277813105 | | 2000 | 5.7307521884774 | | 1999 | 5.2150440673713 | | 1998 | 6.3932031142612 | | 1997 | 7.5658309067403 | | 1996 | 8.0014830157593 | | 1995 | 8.7157302160744 | | 1994 | 6.2960026305075 | | 1993 | 6.3802916083749 | | 1992 | 6.0773434535255 | | 1991 | 6.7272571313972 | | 1990 | 6.8775275660221 | | 1989 | 6.8425161218046 | | 1988 | 6.9145078351806 | | 1987 | 7.2620950908929 | | 1986 | 7.5820782120566 | | 1985 | 7.2953486019391 | | 1984 | 7.4748349370812 | | 1983 | 7.700593159029 | | 1982 | 7.8429656707083 | | 1981 | 8.3801660415608 | | 1980 | 8.9716804204794 | | 1979 | 8.9601248289523 | | 1978 | 8.9609988164913 | | 1977 | 8.6105376205911 | | 1976 | 9.06024655546 | | 1975 | 8.442047367842 | | 1974 | 9.313036856045 | | 1973 | 10.179192227876 | | 1972 | 9.9571066369942 | | 1971 | 9.2942380166886 |
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