It doesn't matter. This is what's called a meta-analysis or meta-study, which is a study of studies, if you will. It's the first attempt to collect, organize, analyze and interpret information produced by others. The intent is to formulate a research question, then determine whether available literature contains enough information to answer the research question.
The answer is a qualified "yes, there is." And more importantly, even the most biased reviewer would have to conclude that the available literature supports the conclusions. Then comes the inevitable recommendation that researchers always make, which is that further study is warranted.
I read the whole thing, by the way.