| Year | Value (SE) |
|---|---|
| 2006 | 0.6460 (0.0477) |
| 2007 | 0.6140 (0.0653) |
| 2008 | 0.4300 (0.0385) |
| 2009 | 0.5990 (0.0588) |
| 2010 | 0.5790 (0.0472) |
| 2011 | 0.3350 (0.0384) |
| 2012 | 0.2600 (0.0401) |
| 2013 | 0.1260 (0.0246) |
| 2014 | 0.2260 (0.0427) |
| 2016 | 0.2510 (0.0323) |
| 2017 | 0.1450 (0.0225) |
| 2018 | 0.3800 (0.0466) |
| 2019 | 0.2900 (0.0451) |
| 2020 | 0.4710 (0.0666) |
