It must be admitted that it is exceedingly difficult to establish a definite basis for a comparison of our statistics relating to mental disorders and those dealing with the frequency of other diseases in the community. As has been shown, our information on the latter subject, such as it is, has to do only with communicable diseases and the reported death rates. In making an analysis of the reports of mental diseases we are limited almost entirely to the institution population. It is true that these statistics are much more reliable than the others, as we are dealing with a stable population entirely under control. The cases, furthermore, are almost invariably subject to a prolonged observation and careful study. The diagnosis in almost every instance is based on elaborate mental examinations and exhaustive personal and family histories. It is, of course, true that there are innumerable cases of mental diseases outside of institutions. There were 18,268 patients at home on visit from the state hospitals alone on January 1, 1920. Those not requiring hospital treatment or custody in an institution can, however, be eliminated for the purpose of comparative studies. The fact that an analysis of death rates alone does not throw any light whatever on the frequence of psychoses for reasons already given will, I think, be conceded. For statistical purposes, at least, it may be assumed that the frequence of mental diseases as shown by a study of the hospital population is fairly representative of conditions existing in the community. For purposes of comparison we may contrast the admission rate of mental diseases per 100,000 of the population in Massachusetts in 1920 with the case rate of communicable diseases as follows: Mental diseases 101.7 Chicken pox 138.4 Diphtheria 194.2 German measles 12.5 Gonorrhea 186.7 Measles 830.7 Mumps 154.1 Scarlet fever 265.2 Syphilis 77.2 Tuberculosis, pulmonary 173.1 Tuberculosis, other forms 20.7 Typhoid fever 24.2 Whooping cough 258.3 The total institution population (mental cases) at the end of the year 1920 represented a rate of 395.49 per 100,000 of the population. It should be borne in mind that, with the exception of tuberculosis and syphilis, the communicable diseases reported above represent, as a rule, the total number of cases in the state during the year. Comparative studies should, therefore, be based not on the number of mental cases in the hospitals at any one given time, but on the total number under treatment during the year. This would indicate _an incidence of mental diseases of 566.98 per 100,000 of the population_.