| Year | Value (SE) |
|---|---|
| 2000 | 0.6120 (0.0131) |
| 2001 | 0.4790 (0.0059) |
| 2002 | 0.6530 (0.0093) |
| 2003 | 0.5590 (0.0094) |
| 2004 | 0.4940 (0.0045) |
| 2005 | 0.4030 (0.0046) |
| 2006 | 0.3790 (0.0069) |
| 2007 | 0.5940 (0.0134) |
| 2008 | 0.6460 (0.0170) |
| 2009 | 0.5530 (0.0125) |
| 2010 | 0.5880 (0.0256) |
| 2011 | 0.4930 (0.0216) |
| 2012 | 0.6720 (0.0170) |
| 2013 | 0.6040 (0.0136) |
| 2014 | 0.6650 (0.0182) |
| 2015 | 0.6230 (0.0623) |
| 2016 | 0.5890 (0.0168) |
| 2018 | 0.6000 (0.0135) |
| 2019 | 0.5580 (0.0215) |
| 2020 | 0.5740 (0.0298) |
| 2021 | 0.4900 (0.0260) |
| 2022 | 0.5710 (0.0282) |
| 2023 | 0.4550 (0.0150) |
