| Year | N | Mean | Standard error | Coefficient of variation |
| 2001 | 25 | 20.0 | 2.2 | 5.4% |
| 2001 | 5 | 20.7 | 2.7 | 13.0% |
| 2002 | 5 | 12.0 | 2.1 | 17.6% |
| 2003 | 5 | 22.5 | 1.5 | 6.5% |
| 2004 | 5 | 18.0 | 0.9 | 4.8% |
| 2005 | 5 | 17.5 | 2.3 | 13.2% |
| 2006 | 5 | 28.9 | 4.0 | 13.8% |
| 2007 | 5 | 15.2 | 1.7 | 11.2% |
| 2008 | 5 | 17.2 | 0.5 | 3.2% |
| 2009 | 5 | 14.5 | 0.7 | 4.8% |
| 2010 | 5 | 17.1 | 1.0 | 6.1% |
| 2011 | 5 | 17.1 | 1.1 | 6.6% |
| 2012 | 5 | 19.7 | 1.1 | 5.7% |