| Year | Value (SE) |
|---|---|
| 2000 | 0.2380 (0.0954) |
| 2001 | 0.2030 (0.0163) |
| 2002 | 0.2890 (0.0241) |
| 2003 | 0.2410 (0.0199) |
| 2004 | 0.1420 (0.0250) |
| 2005 | 0.1280 (0.0214) |
| 2006 | 0.1790 (0.0191) |
| 2007 | 0.2620 (0.0152) |
| 2008 | 0.4170 (0.0579) |
| 2009 | 0.2600 (0.0351) |
| 2010 | 0.3670 (0.0483) |
| 2011 | 0.2220 (0.0442) |
| 2012 | 0.2530 (0.0231) |
| 2013 | 0.2230 (0.0414) |
| 2014 | 0.2340 (0.0325) |
| 2015 | 0.1370 (0.0206) |
| 2016 | 0.1820 (0.0165) |
| 2019 | 0.3170 (0.0353) |
| 2020 | 0.2840 (0.0246) |
| 2021 | 0.2800 (0.0714) |
| 2022 | 0.2240 (0.0358) |
| 2023 | 0.3680 (0.0384) |
