Year Number 1996 0.613030731678009 1998 0.575733542442322 2000 0.595203697681427 2002 0.518141508102417 2003 0.349641561508179 2004 0.330648332834244 2005 0.332869827747345 2006 0.331756144762039 2007 0.308105289936066 2008 0.309818059206009 2009 0.323601871728897 2010 0.320087254047394 2011 0.306695103645325 2012 0.282746523618698 2013 0.297252923250198 2014 0.238057717680931 2015 0.242593795061111 2016 0.245254904031754 2017 0.254438191652298 2018 0.249047264456749 2019 0.271763026714325 2020 0.270491451025009 2021 0.273985654115677 2022 0.267183810472488