| Year | Value (SE) |
|---|---|
| 2003 | 0.2460 (0.0088) |
| 2004 | 0.2860 (0.0047) |
| 2005 | 0.3490 (0.0059) |
| 2006 | 0.1630 (0.0086) |
| 2008 | 0.3460 (0.0345) |
| 2009 | 0.3100 (0.0189) |
| 2010 | 0.3220 (0.0184) |
| 2011 | 0.2640 (0.0150) |
| 2013 | 0.5540 (0.0258) |
| 2014 | 0.4970 (0.0351) |
| 2015 | 0.2020 (0.0437) |
| 2019 | 0.4900 (0.0507) |
| 2020 | 0.6220 (0.0615) |
| 2023 | 0.5320 (0.0320) |
