| Year | Value (SE) |
|---|---|
| 2008 | 0.6904 (0.0689) |
| 2009 | 0.7396 (0.0245) |
| 2010 | 0.8217 (0.0446) |
| 2011 | 0.8252 (0.0226) |
| 2012 | 0.7178 (0.0186) |
| 2013 | 0.8519 (0.0672) |
| 2014 | 0.7786 (0.0407) |
| 2015 | 0.9021 (0.0858) |
| 2016 | 0.8759 (0.0223) |
| 2017 | 0.9253 (0.0305) |
| 2019 | 0.8417 (0.0568) |
| 2020 | 0.8794 (0.0856) |
| 2021 | 0.9361 (0.2433) |
