| Year | Value (SE) |
|---|---|
| 2007 | 0.4480 (0.0236) |
| 2008 | 0.2670 (0.0562) |
| 2009 | 0.2620 (0.0390) |
| 2010 | 0.3140 (0.0348) |
| 2011 | 0.3180 (0.0448) |
| 2012 | 0.3060 (0.0207) |
| 2013 | 0.3840 (0.0323) |
| 2014 | 0.5870 (0.0531) |
| 2015 | 0.3910 (0.0444) |
| 2016 | 0.3960 (0.0278) |
| 2017 | 0.3910 (0.0304) |
| 2018 | 0.3300 (0.0459) |
| 2019 | 0.2910 (0.0297) |
| 2020 | 0.4930 (0.0638) |
| 2021 | 0.4730 (0.0901) |
| 2022 | 0.3390 (0.0648) |
| 2023 | 0.3530 (0.0452) |
| 2024 | 0.2330 (0.0489) |
