Thesis: What are the requirements before saying "There is no research on that" and how to phrase it? I'm writing my masters thesis in computer science and I haven't found a good source for evaluation criteria of the protocols I'm looking at in my thesis. 

What are the requirements before I can say "There is nothing on evaluation criteria in the literature." 

Is the fact that I couldn't find any enough? When do I know that I put enough effort into searching?
Is the fact that I couldn't find any enough? When do I know that I put enough effort into searching?


Obviously, there is no way to "prove" that something does not exist, so on a fundamental level that you did not find anything has to be enough. Of course, you need to make evident that you searched in a reasonable way. The guidelines of structured literature reviews have some rules on how to do this, but essentially it boils down to (1) searching all reasonable sources, and (2) searching for all the terms that could reasonably be used to describe what you are looking for.


  how to phrase it?


The usual phrase is "to the best of our knowledge, this methodology has not been used before to (...)".