Review miningFirst page (search results)
The app list a search returns. A keyword's real difficulty is visible here.
The most honest information about a keyword is the result list itself: who is there, how many ratings they have, whether they carry the word in their title or Apple matched them some other way.
If the apps at the top do not carry the word in their titles, that search is being won by brand and behaviour rather than metadata - you do not enter it by adding a word.
rankcusp stores the first page for every tracked keyword and builds its verdict from it. Somebody who dislikes a score can look at the list and argue; having something to argue with is the definition of not being a black box.