| Year | Value (SE) |
|---|---|
| 2000 | 0.3523 (0.0725) |
| 2001 | 0.2651 (0.0083) |
| 2002 | 0.3150 (0.0167) |
| 2003 | 0.3158 (0.0282) |
| 2004 | 0.1976 (0.0188) |
| 2005 | 0.1971 (0.0155) |
| 2006 | 0.2453 (0.0211) |
| 2007 | 0.2851 (0.0197) |
| 2008 | 0.3747 (0.0343) |
| 2009 | 0.2993 (0.0223) |
| 2010 | 0.3683 (0.0579) |
| 2011 | 0.2285 (0.0245) |
| 2012 | 0.3063 (0.0242) |
| 2013 | 0.2065 (0.0265) |
| 2014 | 0.2106 (0.0205) |
| 2015 | 0.2365 (0.0587) |
| 2016 | 0.3020 (0.0218) |
| 2019 | 0.3946 (0.0604) |
| 2020 | 0.4215 (0.1006) |
| 2021 | 0.3124 (0.3379) |
| 2022 | 0.2845 (0.0786) |
| 2023 | 0.5288 (0.0950) |
