| Year | Value (SE) |
|---|---|
| 2003 | 0.2362 (0.0151) |
| 2004 | 0.2311 (0.0077) |
| 2005 | 0.3151 (0.0094) |
| 2006 | 0.1418 (0.0102) |
| 2008 | 0.2540 (0.0325) |
| 2009 | 0.2672 (0.0206) |
| 2010 | 0.3191 (0.0278) |
| 2011 | 0.2622 (0.0226) |
| 2013 | 0.5230 (0.0426) |
| 2014 | 0.3916 (0.0406) |
| 2015 | 0.1467 (0.0413) |
| 2019 | 0.4214 (0.0693) |
| 2020 | 0.5100 (0.1253) |
| 2023 | 0.3421 (0.0463) |
