The report “Comparative Study of Two Computer Mouse Designs” (Cornell Human Factors Laboratory Technical Report RP7992) included the following description of the subjects used in an experiment: Twenty-four Cornell University students and staff (12 males and 12 females) volunteered to participate in the study. Three groups of 4 men and 4 women were selected by their stature to represent the 5th percentile (female 152.1 ± 0.3 cm, male 164.1 ± 0.4 cm), 50th percentile (female 162.4 ± 0.1 cm, male 174.1 ± 0.7 cm), and 95th percentile (female 171.9 ± 0.2 cm, male 185.7 ± 0.6 cm) ranges . . . All subjects reported using their right hand to operate a computer mouse.

This experimental design incorporated direct control and blocking. 

a. Is the potential effect of the extraneous variable stature (height) addressed by blocking or direct control? 

b. Whether the right or left hand is used to operate the mouse was considered to be an extraneous variable. Is the potential effect of this variable addressed by blocking or direct control?