Thought it is clear that those tags can be used for different things I think there is some overlapping.
Since a study can be used as a reference and a reference can be either a book based on some studies or only one study it seems clear that reference and a study can be the same thing.
Learning methods in itself can be pretty vague and can refer to either a book, a collection of software or to learning techniques. On the other hand, reference overlaps the previous tags because the concept of reference overlaps the above-cited meanings of learning-methods save for the techniques part.
Here I don't think there is any overlapping.
My conclusion is that reference-request overlaps with at least to tags. Thus I wonder if we had better not using it.
The learning-methods have already be given a tag guidance whereas the two others have not.
I would suggest the following tag guidance for published-studies:
Ask for published scientific studies which address the problematic asked by the question.
On the other hand, the vagueness in reference-request prevent me from giving a satisfying definition