| Year | Value | | 2014 | 6.3368774796029 | | 2013 | 6.600764740862 | | 2012 | 6.7499735191716 | | 2011 | 6.8328564705241 | | 2010 | 6.9526214643153 | | 2009 | 7.1304916413551 | | 2008 | 7.1761939520867 | | 2007 | 7.5039190572592 | | 2006 | 7.6359672712621 | | 2005 | 7.7050206219377 | | 2004 | 7.8128962168975 | | 2003 | 7.8567509994143 | | 2002 | 7.9425208721937 | | 2001 | 8.0098686385469 | | 2000 | 8.1123123440221 | | 1999 | 8.1035233467577 | | 1998 | 8.3399708460003 | | 1997 | 8.6829174694962 | | 1996 | 9.2394101586616 | | 1995 | 9.6791571755034 | | 1994 | 9.9147225193135 | | 1993 | 10.227946244814 | | 1992 | 10.421656828489 | | 1991 | 10.611225500331 | | 1990 | 10.876377436494 | | 1989 | 10.896911685259 | | 1988 | 10.879201688843 | | 1987 | 10.844558570456 | | 1986 | 10.811132137746 | | 1985 | 10.757215426131 |
|