To the post originator...
First, get a pronounceable name. Personally, I find it a little insulting to view your name as if a chimpanzee has been let loose at a keyboard.
Second, if you want such information, have the courtesy to explain who you are and why and for whom you are "conducting research" and what makes it "non-commercial". Who are "we" who are going to minimise failures?
Third, your methodology appears somewhat disjointed. How are you to obtain meaningful data from your target groups? Current trainees have not yet failed (and, we trust, will not fail); recently licensed ATCOs, by definition, have not failed; and any ex-trainees who did not succeed might not have a dispassionate view of the reasons.
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