| Year | Value (SE) |
|---|---|
| 2008 | 0.8420 (0.0398) |
| 2009 | 0.7230 (0.0124) |
| 2010 | 0.8040 (0.0295) |
| 2011 | 0.8310 (0.0130) |
| 2012 | 0.7620 (0.0134) |
| 2013 | 0.8160 (0.0441) |
| 2014 | 0.7770 (0.0192) |
| 2015 | 0.7730 (0.0217) |
| 2016 | 0.8630 (0.0094) |
| 2017 | 0.9290 (0.0174) |
| 2019 | 0.8400 (0.0259) |
| 2020 | 0.8580 (0.0128) |
| 2021 | 0.7840 (0.0095) |
