| Year | Value (SE) |
|---|---|
| 2002 | 0.2601 (0.0272) |
| 2004 | 0.3469 (0.1193) |
| 2005 | 0.1922 (0.0579) |
| 2007 | 0.2900 (0.0496) |
| 2008 | 0.4454 (0.0486) |
| 2009 | 0.2670 (0.0349) |
| 2010 | 0.1378 (0.0154) |
| 2011 | 0.5969 (0.0165) |
| 2012 | 0.4403 (0.0131) |
| 2013 | 0.4933 (0.0385) |
| 2014 | 0.4778 (0.0380) |
| 2015 | 0.5175 (0.2611) |
