A study of reliability or stability of response, conducted in Alameda County, California by means of two identical questionnaires completed by respondents within approximately two weeks, indicates that three out of four respondents answered at least go per cent of the items identically; only one in fifty did so for as few as 70–79 per cent of the items. Within this rather narrow range, reliability varied with education, age, length of time between questionnaires, and particularly with the “hardness” of the information requested. The small number of responses indicating poor health, marital dissatisfaction, or a psychological problem were more likely to change for the better than responses indicating no such problem were to change for the worse.