ABSTRACT
Recall that the two basic objectives in experimental design are the elimination of systematic bias and the reduction of error (within group or cell) variance. The main reason for within-group variability is individual differences among the subjects. Thus, even though the subjects receive the same treatment, their scores on the dependent variable can differ considerably because of differences on IQ, motivation, socioeconomic status (SES), and so on. One statistical way of reducing error variance is through analysis of covariance, which was discussed in Chapter 8.
