Suppose a state is considering whether to require all public school teachers to have a master's degree. Currently, only 40 percent of the teachers in the state have such a degree.

a. A researcher conducts a cross-sectional analysis comparing test scores of students in the state whose teachers have master's degrees to the test scores of students whose teachers do not. The researcher finds that students whose teachers have master's degrees score significantly higher on standardized tests. Why might such a study be biased?

b. Now, suppose another state conducted an experiment in which 500 students were randomly

selected to be either in a class taught by a teacher with a master's degree (treatment group) or a class with a teacher without a master's degree (control group).

This experiment found that the treatment group scored significantly better than the control group. How practical is this experiment in informing the state's decision about whether to have a master's degree requirement?