1. Group Comparison Research
Causal-Comparative
(Ex post facto)
Research

2. Purpose of causal comparative research
Attempts to determine cause for
Existing conditions
Preexisting differences in groups
Alleged cause and effect have already occurred
Orientations
Retrospective (basic): starts with an effect and seeks possible causes
Prospective (variation): starts with a cause and investigates its effects on some variable

3. Causal-comparative (ex post facto) research
The independent variable (IV) is not manipulated; it has already occurred
Independent variables sometimes called “attribute variables”
Less costly and time-consuming to conduct
Establishing cause-effect relationships is more difficult than in experiments

4. Procedures in causal-comparative research
Identify an existing condition or event (e.g., differences in socialization among 1st grade students)
Look “backwards” to see what may have caused this difference/condition to occur (i.e., some attended preschool, some did not)
Rule out other causal factors

5. Sometimes confused with correlational research:
Both lack manipulation of variables
Both require caution in interpreting results
Both can support subsequent experimental research

6. Causal comparative vs. correlational research
Attempts to identify cause-effect relationships
At least one independent variable
Two or more groups
Involves a comparison
Correlational
No attempt to understand cause and effect
Two or more variables
Only one group

7. Sometimes confused with experimental research:
Both try to establish cause-effect relationships
Both can test hypotheses concerning the relationship between an independent (X) and a dependent variable (Y)
Both involve group comparisons

8. Comparison to experiments
Causal comparative
Individuals already in groups before study begins
Independent variable has already occurred
Independent variable is not manipulated
Cannot be
Should not be
Could be, but is not
Experiment
Individuals randomly assigned to groups (e.g., treatment or control)
Independent variable manipulated by the researcher

9. Examples of non-manipulated independent variables
Age
Sex
Ethnicity
“Learning style”
Socioeconomic status (SES)
Parent educational level
Family environment
Type of school attended

10. Design of causal-comparative research
Select 2 groups that differ on some IV
One group possesses a characteristic that the other does not
Each group possesses the characteristic, but in differing amounts
Randomly sample Ss from each group
Collect info on Ss to determine equality of the groups
Compare groups on the DV

11. Difficulty in interpreting findings
Establishing cause and effect requires caution!
Alternative explanations:
Different causal variable
Order of causation
Reverse causality
Order of occurrence

12. Evidence necessary to demonstrate that X causes Y:
Establish statistical relationship between X and Y (i.e., correlational research);
determine that X precedes Y in time (collect data over time, i.e., longitudinal research);
demonstrate that other, unknown factors did not determine the dependent variable (i.e., experimental research).

13. Becker & Gersten (1982): “Effects of Project Follow-Through…”
Quasi-experimental study
Ex post facto study
Problem: Are the two groups in this study comparable to one another?

14. In order to make sure that the two groups are comparable, and to ensure that the only post-test differences between the groups are due to the independent variable (the Follow-Through intervention), data were obtained on students’:
family income
gender
language spoken in home
mother’s education
ethnicity
number of siblings.

15. Research Design FOLLOW-THROUGH Year 1 (1975) Gr 5 Year 2 (1976) Gr 6
Site 1
Site 2
Site 3
Site 4
Site 5
Year 2 (1976) Gr 6
NO FOLLOW-THROUGH
Year 1 (1975) Gr 5
Site 1
Site 2
Site 3
Site 4
Site 5
Year 2 (1976) Gr 6

16. Dependent variables Wide-Range Achievement Test (WRAT)
reading
mathematics
Metropolitan Achievement Test (MAT)

17. RESULTS
A total of 180 comparisons of FT to No-FT students. Of these, only 56 (31%) favored FT students!
Largest differences between FT and No-FT students were in basic skills areas.
FT students’ achievement declined by grades 5 and 6 (2-3 years after end of FT).

18. Critique of this research
What are the strengths of the study?
Groups are comparable to one another.
Contrasted statistical with practical significance.
Large sample size.
Multiple “replications” of treatment effect.
What are the weaknesses of the study?
Lack of random assignment.
Focus on standardized test performance.