| Year | Value (SE) |
|---|---|
| 2000 | 0.4400 (0.0724) |
| 2001 | 0.4090 (0.0239) |
| 2002 | 0.4320 (0.0211) |
| 2003 | 0.5460 (0.0308) |
| 2004 | 0.3990 (0.0489) |
| 2005 | 0.3550 (0.0426) |
| 2006 | 0.3800 (0.0303) |
| 2007 | 0.5610 (0.0196) |
| 2008 | 0.5900 (0.0580) |
| 2009 | 0.3910 (0.0341) |
| 2010 | 0.5590 (0.0556) |
| 2011 | 0.3590 (0.0397) |
| 2012 | 0.4640 (0.0271) |
| 2013 | 0.5750 (0.0636) |
| 2014 | 0.4820 (0.0523) |
| 2015 | 0.2730 (0.0382) |
| 2016 | 0.3860 (0.0343) |
| 2019 | 0.3760 (0.0405) |
| 2020 | 0.4280 (0.0521) |
| 2021 | 0.5950 (0.1853) |
| 2022 | 0.2260 (0.0528) |
| 2023 | 0.2760 (0.0294) |
