Definition, incidence, causes (genetics & environmental), studies of twins, expressed emotion, stress.
The CDC website which, supposedly covers most diseases, is a confusing array of diseases. But, go to the alphabetic index, try S for Schizophrenia, and find nothing. Try Mental Health after a few other futile attempts, and you find some statistics about releases for mental patients, numbers of suicides for the year, and a list of nursing homes and other facilities, but no statistics on schizophrenia. It simply may not be a reportable disease. The reasons could be that there are other symptoms, unrelated to this so-called split personality syndrome, and so there is no simple statistic just for Schizophrenia.