| Year | Value (SE) |
|---|---|
| 2004 | 0.1420 (0.0456) |
| 2005 | 0.0460 (0.0125) |
| 2006 | 0.0670 (0.0120) |
| 2007 | 0.1130 (0.0202) |
| 2008 | 0.2250 (0.0200) |
| 2009 | 0.1180 (0.0196) |
| 2010 | 0.1860 (0.0229) |
| 2011 | 0.1820 (0.0628) |
| 2012 | 0.0890 (0.0212) |
| 2013 | 0.0990 (0.0656) |
| 2014 | 0.2450 (0.0585) |
| 2015 | 0.1610 (0.0800) |
