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March, 1954, was the effective starting date of a five-year study on some of the effects of housing quality on health and family life. The actual collection of data began April, 1955, but because of the long-range nature of the study findings that are relevant to the hypotheses will not be available for a number of months. This paper is concerned with the background of the project and with a description of the study methods being employed. As we discuss its different phases, we will have occasion to mention problems incurred in carrying out a controlled study design in which the independent variable is an aspect of the human social environment. These problems rarely get a public airing at a time when they are uppermost in the minds of investigators. When final reports are written early difficulties usually have receded, or are over

N SURVEYS involving natural clusters, I such as families, school classes, sororities, villages, health districts, and the like, the problem of intraclass correlation is frequently ignored, with the likely consequence of overestimating the precision and statistical significance of results obtained. The purpose of this paper is to review briefly some circumstances in which correlation within clusters has been encountered and the various statistical methods that have been used to deal with the problem. Special reference will be made to a study of the housing environment and family life,*1-4 a longitudinal assessment of the effects of housing on morbidity and mental health. In this study, with which the authors are associated, the problem of intraclass correlation arose in the statistical analysis of morbidity data and made a substantial