| Year | Value (SE) |
|---|---|
| 2008 | 0.8340 (0.1635) |
| 2009 | 0.8600 (0.0797) |
| 2010 | 0.8840 (0.0989) |
| 2011 | 0.7710 (0.0621) |
| 2012 | 0.9390 (0.0969) |
| 2013 | 0.8700 (0.1305) |
| 2014 | 0.8650 (0.1335) |
| 2015 | 0.7190 (0.1754) |
| 2016 | 1.0910 (0.0707) |
| 2017 | 0.9500 (0.1269) |
| 2019 | 1.1880 (0.3988) |
| 2020 | 0.7260 (0.2427) |
| 2021 | 0.4210 (0.4143) |
