| Year | Value (SE) |
|---|---|
| 2000 | 0.4270 (0.0143) |
| 2001 | 0.5190 (0.0038) |
| 2002 | 0.4060 (0.0077) |
| 2003 | 0.3500 (0.0081) |
| 2004 | 0.2600 (0.0036) |
| 2005 | 0.2330 (0.0032) |
| 2006 | 0.3090 (0.0069) |
| 2007 | 0.3400 (0.0074) |
| 2008 | 0.4550 (0.0086) |
| 2009 | 0.3700 (0.0064) |
| 2010 | 0.4460 (0.0201) |
| 2011 | 0.3000 (0.0068) |
| 2012 | 0.3770 (0.0093) |
| 2013 | 0.2190 (0.0070) |
| 2014 | 0.2640 (0.0054) |
| 2015 | 0.2650 (0.0165) |
| 2016 | 0.3730 (0.0052) |
| 2019 | 0.4550 (0.0181) |
| 2020 | 0.5410 (0.0152) |
| 2021 | 0.5190 (0.0170) |
| 2022 | 0.5310 (0.0161) |
| 2023 | 0.5940 (0.0101) |
