| Year | Value | | 2017 | 5.2809708941022 | | 2016 | 5.2372746633787 | | 2015 | 5.2646883910507 | | 2014 | 5.3763575042639 | | 2013 | 5.3413949099404 | | 2012 | 5.3957760997974 | | 2011 | 5.3930125336577 | | 2010 | 5.4381705862827 | | 2009 | 5.5157778314389 | | 2008 | 5.5866934050541 | | 2007 | 5.6615921950173 | | 2006 | 5.6816974353187 | | 2005 | 5.7381256736666 | | 2004 | 5.8434985486393 | | 2003 | 5.8995095664834 | | 2002 | 5.9845407935133 | | 2001 | 6.1551063790761 | | 2000 | 6.1323484223058 | | 1999 | 6.5618403331169 | | 1998 | 6.6900788261218 | | 1997 | 6.7220546029585 | | 1996 | 6.8535672209362 | | 1995 | 7.0811247699312 | | 1994 | 7.2685187988646 | | 1993 | 7.2212151141351 | | 1992 | 6.205355263945 | | 1991 | 6.5388784360212 | | 1990 | 6.3552666268573 | | 1985 | 8.1892286198345 | | 1980 | 8.4077866905655 | | 1970 | 9.0833345022075 | | 1960 | 8.8814545667438 |
|