As an example, my daughter was raised as a Jehovah's Witness. She began to have mental health issues by age fourteen and so wasn't qualified for baptism, if she had been baptized she would have been disfellowshipped for suicide attempts. She would not have counted in any statistical study of the congregation, they could say they didn't have any mentally ill in the congregation and been truthful, but their practices make that kind of statistical analysis meaningless.
As another example, I knew of a Jehovah's Witnesses who began hearing voices, a sign of schizophrenia. He was disfellowshipped, as they assumed he was demonized.
The study is meaningless because they shoot their wounded.