Inappropriate Institutionalization

One of the claimed arguments for reform of involuntary commitment laws in the 1960s was that people were being locked up in mental hospitals who were not actually mentally ill, but simply eccentric or inconvenient. I don't find it impossible to believe, and at least one of the landmark cases, O'Connor v. Donaldson (1975), was such a case. The parents arranged to get their adult son hospitalized with no apparent evidence of mental illness.

How common was this? If there were large numbers of such cases, then the reform movement made sense. Does anyone know of any statistical measures of the number of such inappropriate institutionalization cases there were?