two three four tags, user-research, usability-study, usability-testing and testing. The tag wiki for testing is
a technique used to evaluate a product by testing it on users making it clearly about Usability Testing.
All of these fall under User Research. The latter three have been used specifically for usability studies. There definitely needs to be some merges/aliases here.
Which tags are distinct and which should be the master tag(s)? IMO all except User Research need to be combined into a single tag regarding usability studies. I'm less sure whether User Research should be used as a separate but more general tag to include things like Card Sorting that aren't really usability studies. But we definitely don't need all 4 of these, do we?