| Years | Current Study§ | Previous study by Hassan and Schneider (2016)¨ |
|
| MIMIC | MIMIC |
| 2000 | 21.43% | 27.33% |
| 2001 | 21.43% | 26.93% |
| 2002 | 21.43% | 27.20% |
| 2003 | 24.27% | 27.13% |
| 2004 | 24.06% | 26.25% |
| 2005 | 25.44% | 21.47% |
| 2006 | 24.19% | 21.17% |
| 2007 | 21.82% | 21.18% |
| 2008 | 21.15% | 20.69% |
| 2009 | 21.17% | 19.91% |
| 2010 | 20.04% | 19.97% |
| 2011 | 20.27% | 17.94% |
| 2012 | 20.13% | 20.05% |
| 2013 | 20.09% | 21.43% |
| 2014 | 20.21% | N/A |
| 2015 | 19.84% | N/A |
| 2016 | 19.68% | N/A |
| 2017 | 19.10% | N/A |