| Year | Value (SE) |
|---|---|
| 2008 | 0.6400 (0.0199) |
| 2009 | 0.8280 (0.0091) |
| 2011 | 0.6890 (0.0085) |
| 2012 | 0.6550 (0.0124) |
| 2013 | 0.6440 (0.0226) |
| 2014 | 0.7900 (0.0235) |
| 2015 | 0.7830 (0.0422) |
| 2016 | 0.8250 (0.0147) |
| 2018 | 0.7270 (0.0167) |
| 2019 | 0.8010 (0.0095) |
| 2020 | 0.7690 (0.0106) |
| 2021 | 0.8200 (0.0182) |
| 2022 | 0.7220 (0.0114) |
| 2023 | 0.8400 (0.0218) |
