1. Experiments • Explanatory research • True experiments • Experimental designs • Quasi-experimental designs • Internal validity • External validity • Ethical issues • Strengths and weaknesses

2. Explanatory Research • Purpose: to explain, to determine cause/effect • What is explained? Variation in the dependent variable • What can be studied in an experiment? Limited, narrow causal relationships Variables that can be studied in lab Topics for which theory has been developed

3. True experiment includes • Two groups (experimental and control) • Random assignment to groups • Variation in independent variable (manipulated by researcher) • Measurement of dependent variable

4. The groups • Experimental group – is exposed to independent variable (I.V.) • Control group - is not exposed to I.V. • I.V. is the only difference between the groups • Any differences in dependent variable (D.V.) must be due to I.V.

5. Assignment to groups • Randomization • Easy to carry out • Can control for unmeasured or uncontrolled factors • Matching • Specific characteristics matched in both groups • May be very precise • Requires knowledge of relevant characteristics • May not control for omitted factors

6. Pretesting • Measures D.V. before experiment • Establishes comparability of experimental and control groups • Provides baseline for comparison with posttest • May teach or “clue in” subjects (pretest effect) • Costs extra

7. Experimental Designs Effect of I.V. = (O3-O1) – (O4-O2) Classic Pretest-Posttest-Control-Group

8. Experimental Designs Effect of I.V. = (O1-O2) Eliminates effect of pretest Posttest-Only

9. Experimental Designs Effect of I.V. =(O3-O1) – (O4-O2) or (O5-O6) Effect of pretest = (O3-O5) or (O4-O6) Solomon four-group

10. Quasi-Experimental Designs • May be used when true experiment isn’t possible • Usually involve fewer controls • No control group • Approximately equivalent control group • May take place in the field • May be “ex post facto:” designed after the “treatment”

11. Internal Validity

12. External Validity • Generalization to “real world” • Often a problem in experiments • 2 main issues • Would sample subjects behave same way outside lab? • Cross-population generalizability: would findings hold for different groups, times, places?

13. Ethical Issues • Deception (misleading subjects about purpose of experiment) • Selective distribution of benefits (also risks, harm)

14. Strengths Isolation of cause/effect High internal validity Easy to replicate Best used for explanatory studies (testing of hypotheses) Weaknesses External validity may be low or undetermined Ethical issues High cost per subject Experiments’ Strengths and Weaknesses