In an experiment reported by J. Singer and D. Andrade in the article "Regression Models for the Analysis of Pretest/Posttest Data" (Biometrics, Vol. 53, pp. 729-735), the effect of using either a conventional or experimental (hugger) toothbrush was investigated. Twelve female and 12 male preschoolers were selected. Within each gender group, six were randomly assigned to the conventional toothbrush and the remaining six to the experimental toothbrush. After each subject brushed with the assigned toothbrush, a dental plaque index was measured. The higher the dental plaque index, the greater was the amount of plaque on an individual's teeth.

a. Is the statistical design described here a completely randomized design or a randomized block design? Explain your answer.

b. If the statistical design is a randomized block design, what are the blocks?