| Year | Value (SE) |
|---|---|
| 2006 | 0.7670 (0.0250) |
| 2007 | 0.6440 (0.0802) |
| 2008 | 0.4890 (0.0377) |
| 2009 | 0.7930 (0.0371) |
| 2010 | 0.8050 (0.1092) |
| 2011 | 0.5040 (0.0158) |
| 2012 | 0.2160 (0.0237) |
| 2013 | 0.2140 (0.1182) |
| 2014 | 0.2760 (0.0593) |
| 2016 | 0.2850 (0.0318) |
| 2017 | 0.1930 (0.0175) |
| 2018 | 0.4980 (0.0654) |
| 2019 | 0.4030 (0.0507) |
| 2020 | 0.6490 (0.5666) |
