Transcript Causal-comparative (ex post facto) research

Group Comparison Research
Causal-Comparative
(Ex post facto)
Research
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
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
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
Sometimes confused with
correlational research:
• Both lack manipulation of variables
• Both require caution in interpreting results
• Both can support subsequent experimental
research
Causal comparative vs. correlational
research
• Causal comparative
– 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
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
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
Examples of non-manipulated
independent variables
• Age
• Sex
• Ethnicity
• “Learning style”
• Socioeconomic status (SES)
• Parent educational level
• Family environment
• Type of school attended
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
Difficulty in interpreting findings
• Establishing cause and effect requires
caution!
• Alternative explanations:
– Different causal variable
– Order of causation
• Reverse causality
• Order of occurrence
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).
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?
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.
Research Design
• FOLLOW-THROUGH
• Year 1 (1975) Gr 5
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Site 1
Site 2
Site 3
Site 4
Site 5
• Year 2 (1976) Gr 6
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Site 1
Site 2
Site 3
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Site 5
• NO FOLLOW-THROUGH
• Year 1 (1975) Gr 5
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Site 1
Site 2
Site 3
Site 4
Site 5
• Year 2 (1976) Gr 6
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Site 2
Site 3
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Site 5
Dependent variables
• Wide-Range Achievement Test (WRAT)
– reading
– mathematics
• Metropolitan Achievement Test (MAT)
– reading
– mathematics
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).
Critique of this research
• What are the strengths of the study?
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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.