| Year | Value (SE) |
|---|---|
| 2008 | 0.7800 (0.1084) |
| 2009 | 0.6220 (0.0460) |
| 2010 | 0.5430 (0.0376) |
| 2011 | 0.6130 (0.0497) |
| 2012 | 0.5410 (0.0358) |
| 2013 | 0.4570 (0.0353) |
| 2014 | 0.4830 (0.0405) |
| 2015 | 0.4640 (0.0502) |
| 2016 | 0.5420 (0.0382) |
| 2017 | 0.5400 (0.0329) |
| 2018 | 0.5940 (0.0441) |
| 2019 | 0.7400 (0.0616) |
| 2020 | 0.7930 (0.0754) |
| 2021 | 0.5620 (0.1102) |
| 2022 | 0.5860 (0.1362) |
| 2023 | 0.4510 (0.0404) |
| 2024 | 0.4920 (0.1112) |
