| Year | Value (SE) |
|---|---|
| 2002 | 0.3680 (0.0253) |
| 2004 | 0.4180 (0.0158) |
| 2005 | 0.4380 (0.0244) |
| 2007 | 0.4200 (0.0282) |
| 2008 | 0.4380 (0.0272) |
| 2009 | 0.2620 (0.0124) |
| 2010 | 0.1520 (0.0055) |
| 2011 | 0.7200 (0.0071) |
| 2012 | 0.5710 (0.0065) |
| 2013 | 0.5440 (0.0067) |
| 2014 | 0.5260 (0.0171) |
| 2015 | 0.4170 (0.1145) |
