2007 22.8
2008 22.8
2009 22.6
2010 22.3
2011 22.4
2012 22.7
2013 22.1
2014 22.3
2015 21.8
2016 21.1
2017 20.4
2018 20.1
2019 19.2
2020 18.3
2021 17.6
2022 17.7
2023 17.6
