## ANSWERS TO CHAPTER 14 EXERCISES

1. Suppose that the means for the treatment and no-treatment conditions are the same. If so, which requirement of establishing causality has not been met?

Covariation.

2. If the study does not manipulate the treatment, which requirement of establishing causality will be difficult to meet?

Temporal precedence

3. If participants are not randomly assigned to condition, which requirement for establishing causality will be almost impossible to meet?

Ruling out spuriousness

4. Compare and contrast how single-subject experiments and randomized experiments account for non-treatment factors.

 Single-n experiments Randomized experiments 1. Eliminate between subject variables by studying a single subject. 1. Independent random assignment to be sure that irrelevant variables vary randomly rather than systematically. 2. Control relevant environmental factors and demonstrate control of extraneous variables by establishing a stable baseline. 2. Use tests of statistical significance to see if it is unlikely that random factors could account for the differences.

5. What arguments can you make for generalizing results from the single-subject experiment?

They often have strong generalizability, especially when they study well understood, basic, universal processes such as perception and reinforcement. In addition, single-n researchers often repeat their study to establish that the same pattern occurs for several individuals.

6. How do the A-B design and the pretest-posttest design differ in terms of

a. Procedure?

The pretest-posttest design uses more participants, does not attempt to develop a stable baseline, and usually exerts less control over non-treatment variables.

b. Internal validity?

Because the pretest-posttest researcher has not established a stable baseline and does not exert as much control over extraneous variables, the pretest-posttest has less internal validity than the A-B design.

7. How does the single-n researcher’s A–B–A design differ from the quasi-experimenter’s reversal time-series design in terms of

a. Procedure?

The quasi-experimenter studied more participants, but did not establish a stable baseline and did not have as much control over extraneous variables.

b. Internal validity?

The single-n researcher might have more internal validity because of more control over extraneous variables.

8. Design a quasi-experiment that looks at the effects of a course on simulating parenthood, including an assignment that involves taking care of an egg, on changing the expectations of junior-high school students about parenting. What kind of design would you use? Why?

A randomized experiment would probably be the best choice because it is (a) feasible and (b) would have internal validity. The next best choice would probably be a time-series design with a control group because the control group might be able to rule out some of the history effects. A time-series design without a control group would be better than a pretest-posttest design because it could better estimate the effects of maturation. However, a pretest-posttest design would be better than a nonequivalent control group design because the nonequivalent control group is so vulnerable to selection.

9. An ad depicts a student who has improved his grade point average from 2.0 to 3.2 after a stint in the military. Consider Campbell & Stanley's "spurious eight." Is the military the only possible explanation for the improvement?

No—maturation (growing up) and history (other events in the person's life) are also likely possibilities. Instrumentation (grade inflation, transferring to an easier school or major) is also a possibility.

10. According to one study, holding students back a grade harmed students. The evidence: students who had been held back a grade did much worse in school than students who had not been held back.

a. Does this evidence prove that holding students back harms their performance? Why or why not?

No—there is a strong possibility that those who were held back differ in certain ways from those who were not held back.

b. If you were a researcher hired by the Dept. of Education to test the assertion that holding students back harms them, which of the designs in this chapter would you use? Why?

A time series design would be inadequate because dropping out could reflect some historical force (better employment opportunities). A nonequivalent control group would not be adequate because the groups are different to begin with. Therefore, you should use a two-group time series design. To make your “held back” group and “not held back” groups as equivalent as possible, you might

attempt to match on key variables, such as IQ and attendance.

hope that you could find a district where students were held back according to some rule (scored below 50% on a standardized test). Then, you might compare those who were just above the cut-off (50-51%) to those who were just below (49-50%).

hope that different districts had different cut-off points so that you could compare 50% scorers who were held back against 50% scorers who advanced.