The uses of data on siblings for educational and psychological research purposes are explained. Examples are based on empirical data. Five types of research problems, using sibling data, are explored: (1) statistical control of family background variance in psychological or educational studies which use analysis of covariance (ANCOVA) or multiple regression analysis; (2) testing the adequacy of age-standardized scores; (3) testing the interval scale property of age standardized scores, such as intelligence or achievement quotients; (4) analysis of the correlations among variables into components which can be attributed to correlation among variables between families or to correlation among variables within families; and (5) assessment of the cultural bias in test scores or of the common factors measured by tests. Between-family-variance is defined as variance due to genetic and environmental influences common to all siblings in a family, but which differ between families. Variance within families is described as genetic and environmental influences which cause siblings to differ from one another. Some types of variables can have both between-family and within-family differences. (MH)