Research is incredibly hard with education.  Consider the following: all of the people you have met with a university degree have learned, by some definition, the material in their degree.  Yet, when we interview people for jobs we clearly find that a very large proportion of computer programmers did not learn the material at school to a level which would allow them to use it.<p>I taught English as a foreign language for 5 years.  But I also have the experience of taking more than 10 years of French lessons.  I can not, nor have I ever been able to speak French.  And yet, I passed more than 10 1-year courses on the subject.  In the 5 years that I taught English -- to literally thousands of students, some over the period of 3 years, none of them learned how to speak English as a result of my teaching.  Although, I was praised for my ability to teach.<p>I speak Japanese fluently.  I learned it on my own as an adult.  I have never taken a Japanese course and nobody ever taught me Japanese.  And yet, I managed to survive 2 years of married life without once speaking English (my wife has since learned to speak English).<p>So the not-so-trivial question is: how do you measure learning?  And how do you assign that learning to different teaching techniques?  Not only are the papers generally quite poor, the thing they are trying to measure is incredibly elusive.  It is a breeding ground for self deception and outright fraud.