Presentation on theme: "Group Comparison Research"— Presentation transcript:
1 Group Comparison Research Causal-Comparative(Ex post facto)Research
2 Purpose of causal comparative research Attempts to determine cause forExisting conditionsPreexisting differences in groupsAlleged cause and effect have already occurredOrientationsRetrospective (basic): starts with an effect and seeks possible causesProspective (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 occurredIndependent variables sometimes called “attribute variables”Less costly and time-consuming to conductEstablishing 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 variablesBoth require caution in interpreting resultsBoth can support subsequent experimental research
6 Causal comparative vs. correlational research Attempts to identify cause-effect relationshipsAt least one independent variableTwo or more groupsInvolves a comparisonCorrelationalNo attempt to understand cause and effectTwo or more variablesOnly one group
7 Sometimes confused with experimental research: Both try to establish cause-effect relationshipsBoth can test hypotheses concerning the relationship between an independent (X) and a dependent variable (Y)Both involve group comparisons
8 Comparison to experiments Causal comparativeIndividuals already in groups before study beginsIndependent variable has already occurredIndependent variable is not manipulatedCannot beShould not beCould be, but is notExperimentIndividuals randomly assigned to groups (e.g., treatment or control)Independent variable manipulated by the researcher
9 Examples of non-manipulated independent variables AgeSexEthnicity“Learning style”Socioeconomic status (SES)Parent educational levelFamily environmentType of school attended
10 Design of causal-comparative research Select 2 groups that differ on some IVOne group possesses a characteristic that the other does notEach group possesses the characteristic, but in differing amountsRandomly sample Ss from each groupCollect info on Ss to determine equality of the groupsCompare groups on the DV
11 Difficulty in interpreting findings Establishing cause and effect requires caution!Alternative explanations:Different causal variableOrder of causationReverse causalityOrder 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 studyEx post facto studyProblem: 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 incomegenderlanguage spoken in homemother’s educationethnicitynumber of siblings.
15 Research Design FOLLOW-THROUGH Year 1 (1975) Gr 5 Year 2 (1976) Gr 6 Site 1Site 2Site 3Site 4Site 5Year 2 (1976) Gr 6NO FOLLOW-THROUGHYear 1 (1975) Gr 5Site 1Site 2Site 3Site 4Site 5Year 2 (1976) Gr 6
16 Dependent variables Wide-Range Achievement Test (WRAT) readingmathematicsMetropolitan Achievement Test (MAT)
17 RESULTSA 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.