| Year | Value (SE) |
|---|---|
| 2004 | 0.0930 (0.0107) |
| 2005 | 0.0690 (0.0089) |
| 2006 | 0.0640 (0.0066) |
| 2007 | 0.1830 (0.0313) |
| 2008 | 0.2510 (0.0183) |
| 2009 | 0.1490 (0.0186) |
| 2010 | 0.1500 (0.0136) |
| 2011 | 0.1000 (0.0137) |
| 2012 | 0.1200 (0.0257) |
| 2013 | 0.0580 (0.0152) |
| 2014 | 0.2320 (0.0403) |
| 2015 | 0.1260 (0.0280) |
