2007 18.1
2008 16.1
2009 16.7
2010 16.2
2011 17.6
2012 18.7
2013 17.8
2014 14.9
2015 14.1
2016 14.8
2017 13.5
2018 11.9
2019 9.6
2020 9.3
2021 9.3
2022 9.2
2023 9.2
