| 2011 | 2012 | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 |
| 0.68 | 0.67 | 0.66 | 0.68 | 0.67 | 0.66 | 0.66 | 0.66 | 0.64 | 0.59 | 0.62 |