| Year | Value (SE) |
|---|---|
| 2006 | 0.8418 (0.0807) |
| 2007 | 0.9523 (0.2018) |
| 2008 | 0.8774 (0.1239) |
| 2009 | 0.7552 (0.0996) |
| 2010 | 0.7197 (0.1260) |
| 2011 | 0.6644 (0.0921) |
| 2012 | 1.2048 (0.2936) |
| 2013 | 0.5876 (0.3958) |
| 2014 | 0.8193 (0.2836) |
| 2016 | 0.8823 (0.1735) |
| 2017 | 0.7503 (0.1998) |
| 2018 | 0.7632 (0.1922) |
| 2019 | 0.7195 (0.2198) |
| 2020 | 0.7263 (0.6852) |
