| year | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 | 2011 | 2012 | 2013 | sum | ratio |
| Theoretical study | 6 | 18 | 15 | 22 | 32 | 30 | 49 | 51 | 46 | 43 | 61 | 373 | 42.9% |
| Experience study | 18 | 25 | 24 | 29 | 30 | 30 | 42 | 40 | 41 | 51 | 53 | 383 | 44.1% |
| Comparison study | 7 | 0 | 7 | 4 | 10 | 6 | 2 | 4 | 5 | 6 | 3 | 54 | 6.2% |
| Empirical research | 3 | 6 | 5 | 4 | 9 | 7 | 3 | 5 | 9 | 3 | 5 | 59 | 6.8% |