| Year | Value | | 2020 | 0.31998316127378 | | 2019 | 0.32313689798599 | | 2018 | 0.3302626254798 | | 2017 | 0.32904170637872 | | 2016 | 0.34979107601988 | | 2015 | 0.35631426779732 | | 2014 | 0.36278327922918 | | 2013 | 0.36959425125123 | | 2012 | 0.37664662720233 | | 2011 | 0.38357561007011 | | 2010 | 0.39020237479998 | | 2009 | 0.39628715099001 | | 2008 | 0.40071473728897 | | 2007 | 0.40476765009027 | | 2006 | 0.39666217307738 | | 2005 | 0.47478628588103 | | 2004 | 0.48308398661461 | | 2003 | 0.37150985977718 | | 2002 | 0.40350671928378 | | 2001 | 0.41364970469399 | | 2000 | 0.29053675966515 | | 1999 | 0.41909641374053 | | 1998 | 0.38292223367624 | | 1997 | 0.39387291569237 | | 1996 | 0.39653890785453 | | 1995 | 0.4154124398654 | | 1994 | 0.28356105711964 | | 1993 | 0.27655212489342 | | 1992 | 0.28467260442283 | | 1991 | 0.29270706932436 |
|