| Year | Value (SE) |
|---|---|
| 2002 | 0.2030 (0.0596) |
| 2004 | 0.3190 (0.2872) |
| 2005 | 0.1550 (0.1406) |
| 2007 | 0.4260 (0.1210) |
| 2008 | 0.3970 (0.0915) |
| 2009 | 0.3980 (0.1425) |
| 2010 | 0.0910 (0.0149) |
| 2011 | 0.4150 (0.0389) |
| 2012 | 0.3380 (0.0217) |
| 2013 | 0.3140 (0.0285) |
| 2014 | 0.3240 (0.0291) |
| 2015 | 0.2640 (0.0828) |
