Sir Ronald Fisher, the father of modern experimental design, extolled the advantages of block designs in his classic book, The Design of Experiments. He observed that block designs enable researchers to reduce error variation and thereby obtain more powerful tests of false null hypotheses. In the behavioral sciences, a significant source of error variation is the nuisance variable of individual differences. This nuisance variable can be isolated by assigning participants or experimental units to blocks so that at the beginning of an experiment, the participants within a block are more homogeneous with respect to the dependent variable than are participants in different blocks. Three procedures are used to form homogeneous blocks.

• Match participants on a variable that is correlated with the dependent variable. Each block consists ...