ERIC Identifier: ED339748 Publication Date: 1991-12-00
Author: Bangert-Drowns, Robert L. - Rudner, Lawrence M.
Source: ERIC Clearinghouse on Tests Measurement and Evaluation
Meta-Analysis in Educational Research. ERIC Digest.
"I had hoped to find research to support or to conclusively oppose my belief
that quality integrated education is the most promising approach. For every
study that contains a recommendation, there is another, equally well documented
study, challenging the conclusions of the first...No one seems to agree with
anyone else's approach. But more distressing: no one seems to know what works."
Senator Fritz Mondale's quote illustrates a common plight. Educational
research often produces contradictory results. Differences among studies in
treatments, settings, measurement instruments, and research methods make
research findings difficult to compare. Even frequent replications can prove
inconclusive. Literature on a topic may be so extensive as to obscure trends
with an overwhelming amount of information.
Meta-analysis is a collection of systematic techniques for resolving apparent
contradictions in research findings. Meta-analysts translate results from
different studies to a common metric and statistically explore relations between
study characteristics and findings.
This digest first describes meta-analysis as a research method. The need and
general approach are discussed. We then identify some common approaches toward
conducting meta-analysis in education and outline their advantages and
META-ANALYSIS AS A RESEARCH METHOD
Gene Glass first used
the term "meta-analysis" in 1976 to refer to a philosophy, not a statistical
technique. Glass argued that literature review should be as systematic as
primary research and should interpret the results of individual studies in the
context of distributions of findings, partially determined by study
characteristics and partially random. Since that time, meta-analysis has become
a widely accepted research tool, encompassing a family of procedures used in a
variety of disciplines. A recent search of the ERIC database identified over 800
articles written after 1980 that use or discuss meta-analysis.
Meta-analysis responds to several problems in educational research. First,
important issues are studied by numerous investigators. The amount of
information on a given topic therefore is often overwhelming and not amenable to
summary. Even when there are relatively few studies on a given topic, it is
difficult to determine if outcome differences are attributable to chance, to
methodological inadequacies, or to systematic differences in study
characteristics. Informal methods of narrative review permit biases to remain
easily undetected. Reviewers' biases can influence decisions about study
inclusion, relative weights given to different findings, and analysis of
relations between study features and outcomes. These biases can have clandestine
effects when reviewers do not systematically seek to reduce them or provide
sufficient information for readers to evaluate their extent.
Meta-analysis typically follows the same steps as primary research. The
meta-analyst first defines the review's purpose. Organizing frameworks can be
practical or theoretical questions of varying scope, but they must be clear
enough to guide study selection and data collection. Second, sample selection
consists of applying specified procedures for locating studies that meet
specified criteria for inclusion. Typically, meta-analyses are comprehensive
reviews of the full population of relevant studies. Third, data are collected
from studies in two ways. Study features are coded according to the objectives
of the review and as checks on threats to validity. Study outcomes are
transformed to a common metric so that they can be compared. A typical metric in
educational research is the effect size, the standardized difference between
treatment and control group means. Finally, statistical procedures are used to
investigate relations among study characteristics and findings.
Criticisms of meta-analysis tend to fall into two categories. Some complain
that meta-analysis obscures important qualitative information by "averaging"
simple numerical representations across studies. Other critics argue that
research is best reviewed by a reflective expert who can sift kernels of insight
from the confusing argumentation of a field.
VOTE-COUNTING -- Some reviews
categorize findings as significantly positive (favoring the treatment group),
significantly negative, or nonsignificant. The category with the most entries is
considered the best representation of research in this area. This as an inexact
approach to integrating research. Vote-counting confuses treatment effect and
sample size because statistical significance is a function of both. Given the
modest power of typical educational research to detect true effects as
statistically significant, conclusions from vote-counting can be very
CLASSIC OR GLASSIAN META-ANALYSIS -- Glass' early meta-analyses set the
pattern for conventional meta-analysis: define questions to be examined, collect
studies, code study features and outcomes, and analyze relations between study
features and outcomes. These early meta-analyses, and later ones following this
tradition, share three distinguishing features. First, "classic" meta-analysis
applies liberal inclusion criteria. Glass argued that one should not disregard
studies on the basis of study quality a priori; a meta-analysis itself can
determine if study quality is related to variance in reported treatment effect.
Second, the unit of analysis is the study finding. A single study can report
many comparisons between groups and subgroups on different criteria. Effect
sizes are calculated for each comparison. Third, meta-analysts using this
approach may average effects from different dependent variables, even when these
measure different constructs.
Glassian meta-analysis has proven quite robust when submitted to critical
re-analysis. Its use of conventional statistical tests render the method and its
results accessible to most educational researchers. However, using study
findings as the units of analysis produces nonindependent data and gives greater
weight to studies with many comparisons. Averaging across constructs and
including studies with obvious methodological flaws can confuse the reliability
STUDY EFFECT META-ANALYSIS -- Study effect meta-analysis alters the Glassian
form in two ways. First, inclusion rules are more selective. Studies with
serious methodological flaws are excluded. Second, the study is the unit of
analysis. One effect size is computed for each study. This preserves the
independence of the data and gives equal weight to all included studies.
Unfortunately, it also reduces the number of data points analyzed in the review.
And, of course, a reviewer's biases may operate in decisions to exclude studies.
TESTS OF HOMOGENEITY -- Some reviewers argue that conventional statistical
tests are inappropriate for meta-analysis. Homogeneity tests were developed to
determine the likelihood that variance among effect sizes is due only to
sampling error. If the homogeneity statistic is significant for a group of
studies, a procedure analogous to analysis of variance can be used. Studies are
repeatedly divided into subgroups according to study features until within-group
variation is nonsignificant.
Numerous factors can cause variation in effect sizes: measurement
unreliability, range restrictions, reporting errors, within-study statistical
adjustments, unreported factors, etc. Homogeneity tests are very likely to
indicate heterogeneity among effect sizes even when the variation is of no
practical or theoretical importance. Successively dividing subgroups according
to these tests can capitalize on chance and cause the incorrect identification
of moderators. Kulik and Kulik defend conventional analysis of variance for
meta-analysis and suggest that homogeneity tests may ignore an important nesting
PSYCHOMETRIC META-ANALYSIS -- Hunter and Schmidt's approach to meta-analysis
combines some of the best features of other approaches. All studies related to a
given topic are gathered, regardless of quality. The distribution of effect
sizes is corrected for sampling error, measurement error, range restriction, and
other systematic artifacts. If the remaining variance is still large, effect
sizes are grouped into subsets according to preselected study features, and each
subset is meta-analyzed separately. Ideally, the meta-analysis should estimate
true treatment effects under conditions typical of those represented in the
studies and predict treatment effects under conditions determined by the
reviewer. Unfortunately, this technique requires substantial information from
individual studies for accurate correction of effect sizes. This information is
not always available in research reports.
Bangert-Drowns, R.L. (1986). Review of
developments in meta-analytic methods. Psychological Bulletin, 99, 388-399.
Glass, G.V, McGaw, B., & M.L. Smith (1981). Meta-analysis in social
research. Beverly Hills, CA: Sage.
Hunter, J.E., & F.L. Schmidt (1990). Methods of meta-analysis. Newbury
Park, CA: Sage.
Kulik, J.A., & C.-L.C. Kulik (1989). Meta-analysis in education.
International Journal of Educational Research, 13, 221-340.
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