| Year | Value (SE) |
|---|---|
| 2002 | 0.1665 (0.0247) |
| 2004 | 0.3386 (0.1931) |
| 2005 | 0.1614 (0.0701) |
| 2007 | 0.2599 (0.0568) |
| 2008 | 0.3394 (0.0655) |
| 2009 | 0.1757 (0.0319) |
| 2010 | 0.1040 (0.0137) |
| 2011 | 0.4691 (0.0262) |
| 2012 | 0.4298 (0.0214) |
| 2013 | 0.3774 (0.0380) |
| 2014 | 0.4157 (0.0397) |
| 2015 | 0.4279 (0.2432) |
