Final answer:
In this experimental design, direct control of extraneous variables is important. Blocking can be used to control for specific variables that may influence the outcome.
Step-by-step explanation:
In this experimental design, it is important to include direct control of extraneous variables to ensure that the only difference between the groups being compared is the treatment. This can be achieved through random assignment of participants to treatment groups. Blocking is a technique used to control for a specific variable that may influence the outcome. It involves dividing participants into subgroups based on that variable and then randomly assigning treatments within each subgroup.