| Year | Value (SE) |
|---|---|
| 2008 | 0.6970 (0.0192) |
| 2009 | 0.7160 (0.0098) |
| 2010 | 0.7850 (0.0320) |
| 2011 | 0.7490 (0.0084) |
| 2012 | 0.7030 (0.0091) |
| 2013 | 0.5860 (0.0164) |
| 2014 | 0.6650 (0.0092) |
| 2015 | 0.5420 (0.0111) |
| 2016 | 0.7540 (0.0083) |
| 2017 | 0.7490 (0.0155) |
| 2018 | 0.6020 (0.0146) |
| 2019 | 0.4510 (0.0093) |
| 2020 | 0.6030 (0.0163) |
| 2021 | 0.6590 (0.0149) |
| 2022 | 0.7730 (0.0165) |
| 2023 | 0.6190 (0.0165) |
