Find the product of the z-scores by multiplying each of the pairs of z-scores (z xz y). To use this formula you would first convert the scores within each variable to z-scores (this could be done with the descriptives procedure in SPSS). Where z x and z y are z scorers, and n = the number of pairs of data The Pearson product-moment correlation is defined the average of the sum of the cross-products of z-scores. It computes the regression equation between pretest and posttest scores for each group in the design and then uses the information from those regressions to answer the question pose above.īefore we get to the ANCOVA model we need to have an understanding of the concepts of correlation and regression.Ī correlation coefficient, r, is a measure of the relationship between two variables. You go to your data file and select participants who have the same range of pretest scores (blocking) or you find a pairs of participants who have the same pretest score (matching). The procedures used in blocking and matching are very mechanical. ANCOVA can be used in either experimental or quasi-experimental designs. Analysis of covariance (ANCOVA) answers the question: What are the differences in the posttest scores if I hold constant the pretest scores? It is a procedure, like blocking and matching, that can be used to control for differences in pretest scores.
