| 2000 | 2 |
| 2005 | 2 |
| 2010 | 1.7 |
| 2011 | 1.7 |
| 2012 | 1.5 |
| 2013 | 1.6 |
| 2014 | 1.7 |
| 2015 | 1.5 |
| 2016 | 1.4 |
| 2017 | 1.5 |
| 2018 | 1.6 |
| 2019 | 1.7 |