| Year | Value (SE) |
|---|---|
| 2000 | 0.2460 (0.0315) |
| 2001 | 0.1980 (0.0110) |
| 2002 | 0.3140 (0.0213) |
| 2003 | 0.1860 (0.0181) |
| 2004 | 0.2100 (0.0280) |
| 2007 | 0.3260 (0.0175) |
| 2010 | 0.2930 (0.0208) |
| 2011 | 0.2970 (0.0250) |
| 2012 | 0.3530 (0.0210) |
| 2013 | 0.3340 (0.0254) |
| 2014 | 0.3340 (0.0271) |
| 2015 | 0.2530 (0.0388) |
| 2016 | 0.3900 (0.0241) |
| 2017 | 0.2630 (0.0211) |
| 2018 | 0.2590 (0.0165) |
| 2022 | 0.2060 (0.0554) |
| 2023 | 0.2070 (0.0209) |
| 2024 | 0.2610 (0.0649) |
