18. Compare the investigation described in Problem 9 with that in Problem 14. Suppose a significant difference had been found in both—in favor of the children who attended preschool in Problem 9 and in favor of Group 2 in Problem 14.

(a) For which investigation would it be easier to clarify the relationship between cause and effect? (Explain.)

(b) What are some other possible explanations—other than whether a child attended preschool—for a significant difference in sociability in Problem 9?