Year Number 1996 0.421715974807739 1998 0.381433337926865 2000 0.396205484867096 2002 0.392881572246552 2003 0.330509543418884 2004 0.316001296043396 2005 0.300349622964859 2006 0.273782998323441 2007 0.262691110372543 2008 0.263591170310974 2009 0.26473930478096 2010 0.260473906993866 2011 0.250866293907166 2012 0.221984326839447 2013 0.228019788861275 2014 0.196386188268661 2015 0.201144516468048 2016 0.206087306141853 2017 0.213935792446136 2018 0.212307974696159 2019 0.229520440101624 2020 0.232469826936722 2021 0.234690770506859 2022 0.233039766550064