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 Classic Pretest-Posttest-Control-Group
Groups
Pretest
I.V.
Uncontrolled factors
Posttest
Change
Exper. O1 X O3
O3-O1
Control O2 O4
O4-O2
Effect of I.V. = (O3-O1) – (O4-O2)

8. Experimental Designs Posttest-Only Effect of I.V. = (O1-O2)
Groups
Pretest
I.V.
Uncontrolled factors
Posttest
Change
Exper.
N/A X O1
Control O2
Effect of I.V. = (O1-O2)
Eliminates effect of pretest

9. Experimental Designs Solomon four-group
Groups
Pretest
I.V.
Uncontrolled factors
Posttest
Change
Exper. 1 O1 X O3
O3-O1
Control 1 O2 O4
O4-O2
Exper. 2 O5
Control 2 O6
Effect of I.V. = (O3-O1) – (O4-O2) or (O5-O6)
Effect of pretest = (O3-O5) or (O4-O6)

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 Source of Invalidity Solution
History – outside events
Control group
Maturation – changes in subjects
Testing – subject may learn
Instrumentation - measurement
Statistical regression - moderation
Selection bias- groups not comparable
Randomization
Mortality – dropping out
Contamination (competition, demoralization)
Treatment misidentification (experimenter expectations, placebo effect, Hawthorne effect)
Randomization, double blind, process analysis

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. Experiments’ Strengths and Weaknesses
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