The criterion for mental illness and depression was that the person had been admitted to a mental hospital or had been treated by a psychiatrist. That is an objective criterion and you do not need to understand mental illness to reach the conclusions he did.
As everyone knows, the religion discourages people from consulting psychologists or psychiatrists, so that you can't reliably use this as criteria for determining mental illness in this religion. Someone who is experienced in these kind of studies and impartial would have figured that out, but Rolf is not impartial and not experienced, he is too close to the subject to see clearly.
Jehovah's Witnesses are told that just about any problem can be fixed by more study, meetings, prayer and field service and to be wary of "worldly" therapists. I know of many Jehovah's Witnesses that were obviously suffering from some kind of mental illness but who never got treatment of any kind. Elders have no training in therapy or counseling and are not qualified to determine if someone needs treatment, and most JWs will consult an elder before going to any kind of therapy.
And your statement that you don't need experience in mental health to interpret statistics on mental health is patently ridiculous. As the Mark Twain quote goes, there are three kinds of lies; lies, damn lies and statistics. Statistics have to be interpreted to be meaningful, and how good can the interpretation be if the person involved has no training in the field and worse, is obviously biased?
This whole study is a fraud in my opinion.