| Year | Value (SE) |
|---|---|
| 2008 | 0.6385 (0.0393) |
| 2009 | 0.6967 (0.0194) |
| 2010 | 0.7316 (0.0461) |
| 2011 | 0.6846 (0.0111) |
| 2012 | 0.6200 (0.0116) |
| 2013 | 0.5608 (0.0243) |
| 2014 | 0.6158 (0.0142) |
| 2015 | 0.6775 (0.0479) |
| 2016 | 0.7548 (0.0173) |
| 2017 | 0.6381 (0.0219) |
| 2018 | 0.5761 (0.0228) |
| 2019 | 0.4050 (0.0142) |
| 2020 | 0.4553 (0.0957) |
| 2021 | 0.3750 (0.1052) |
| 2022 | 0.3633 (0.0482) |
| 2023 | 0.5949 (0.0639) |
