{ "id": "ephvw4r85h", "slug": "meta-regression", "title": "Meta-regression", "updated": "2026-10-15", "topic_path": [ { "id": "physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/topics/physical" }, { "id": "physical.math", "label": "Mathematics and statistics", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.math" }, { "id": "physical.math.statistics", "label": "Statistics and probability", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.math.statistics" }, { "id": "physical.math.statistics.stats_statistical_inference", "label": "Statistical inference, estimation, sampling, and testing", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.math.statistics.stats_statistical_inference" }, { "id": "physical.math.statistics.stats_statistical_inference.regression-analysis", "label": "Regression analysis", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.math.statistics.stats_statistical_inference.regression-analysis" } ], "geo": [ { "id": "geo.nongeo.t1946.physical.math.statistics.stats_statistical_inference.regression-analysis", "label": "Non-geographic · 1946 to 2000: Regression analysis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.math.statistics.stats_statistical_inference.regression-analysis", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1946", "label": "Non-geographic · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946" }, { "id": "geo.nongeo.t1946.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical" }, { "id": "geo.nongeo.t1946.physical.math", "label": "Mathematics and statistics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.math" }, { "id": "geo.nongeo.t1946.physical.math.statistics", "label": "Statistics and probability", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.math.statistics" }, { "id": "geo.nongeo.t1946.physical.math.statistics.stats_statistical_inference", "label": "Statistical inference, estimation, sampling, and testing", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.math.statistics.stats_statistical_inference" }, { "id": "geo.nongeo.t1946.physical.math.statistics.stats_statistical_inference.regression-analysis", "label": "Regression analysis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.math.statistics.stats_statistical_inference.regression-analysis" } ] } ], "excerpt": "Meta-regression is a statistical technique in meta-analysis that regresses the effect sizes of multiple studies on study-level covariates, called moderators, to explain why studies differ.", "snippet": "Meta-regression is a statistical technique in meta-analysis that regresses the effect sizes of multiple studies on study-level covariates, called moderators, to explain why studies differ.", "node": "physical.math.statistics.stats_statistical_inference.regression-analysis", "markdown": "# Meta-regression\n\nMeta-regression is a statistical technique in meta-analysis that regresses the effect sizes of multiple studies on study-level covariates, called moderators or effect modifiers, to explain why studies of the same question reach different findings. It is the continuous, multivariable generalization of subgroup analysis: a two-category subgroup split is mathematically a meta-regression with a single binary covariate, and the usual random-effects pooled estimate is the special case with no covariates at all.[1](https://onlinelibrary.wiley.com/doi/10.1002/jrsm.1338) • [2](https://doing-meta.guide/metareg) • [3](https://www.ovid.com/journals/cesm/fulltext/10.1002/cesm.70099~exploring-heterogeneity-in-meta-analysis-using-trial-level) [Understanding](https://www.edgepedia.org/understanding) the role of moderators was given high priority at the beginning of meta-analysis, and meta-regression was provided as the method for achieving it.[1](https://onlinelibrary.wiley.com/doi/10.1002/jrsm.1338)\n\n| Key fact | Detail |\n|---|---|\n| What it produces | Regression coefficients relating study-level covariates to effect size, plus residual heterogeneity \\( \\tau^2 \\) after adjustment[2](https://doing-meta.guide/metareg) |\n| Underlying model | Mixed-effects regression with two error terms: within-study sampling error and between-study heterogeneity[2](https://doing-meta.guide/metareg) |\n| Estimation | Two steps: estimate \\( \\tau^2 \\), then weighted least squares with weights \\( w_i = 1/(\\hat{\\tau}^2 + v_i) \\)[4](https://cran.r-project.org/web/packages/metafor/vignettes/metafor.pdf) |\n| Study-count rule of thumb | At least 10 studies per covariate[5](https://www.stata.com/manuals/metametaregress.pdf) |\n| Fit measure | Pseudo \\( R^2 = 1 - \\hat{\\tau}^2_{\\text{unexplained}}/\\hat{\\tau}^2_{\\text{total}} \\)[2](https://doing-meta.guide/metareg) |\n| Recommended test | Knapp–Hartung adjustment, which reduces false positives[2](https://doing-meta.guide/metareg) |\n| Structural limitation | Ecological (aggregation) bias: study-level associations need not hold at the patient level[6](https://casrai.org/guides/meta-regression-in-meta-analysis) |\n\n## How it works\n\nThe mixed-effects model writes each observed effect size as a linear function of study-level covariates plus two error terms: \\( \\hat{\\theta}_k = \\theta + \\beta x_k + \\epsilon_k + \\zeta_k \\), where \\( \\epsilon_k \\) is the sampling error through which a study's effect size deviates from its true effect, and \\( \\zeta_k \\) is the between-study heterogeneity term with variance \\( \\tau^2 \\).[2](https://doing-meta.guide/metareg) Here \\( \\tau^2 \\) denotes residual heterogeneity, the variability in true effects not accounted for by the moderators.[7](https://wviechtb.github.io/metafor/reference/metafor-package.html)\n\nThree model families differ in what they assume about that residual term. A fixed-effect meta-regression assumes the moderators account for all between-study heterogeneity; some authors argue it should not be used because in practice the included moderators rarely capture all the heterogeneity, leading to excessive type I errors.[5](https://www.stata.com/manuals/metametaregress.pdf) A random-effects (mixed) meta-regression adds the \\( \\tau^2 \\) component for residual heterogeneity.[5](https://www.stata.com/manuals/metametaregress.pdf) The distinction matters for inference: fixed-effects models condition on the true effects of the included studies, while random-effects models allow generalization to a hypothetical population of studies beyond those included.[8](https://search.r-project.org/CRAN/refmans/metafor/html/misc-models.html) • [9](https://doi.org/10.1037/1082-989x.3.4.486)\n\n## How it is done\n\nFitting is a two-step procedure. First, residual heterogeneity \\( \\tau^2 \\) is estimated with an estimator such as DerSimonian–Laird, Hedges, Hunter–Schmidt, Sidik–Jonkman, maximum likelihood, REML, or empirical Bayes; the DerSimonian–Laird estimator itself was introduced by Rebecca DerSimonian and Nan Laird in 1986.[4](https://cran.r-project.org/web/packages/metafor/vignettes/metafor.pdf) • [10](https://doi.org/10.1016/0197-2456%2886%2990046-2) Second, the coefficients are estimated by weighted least squares with weights \\( w_i = 1/(v_i + \\hat{\\tau}^2) \\), so studies with smaller standard errors receive higher weight.[4](https://cran.r-project.org/web/packages/metafor/vignettes/metafor.pdf) • [2](https://doing-meta.guide/metareg)\n\nCovariates should be pre-specified in the protocol, and post-hoc findings treated as hypothesis-generating.[6](https://casrai.org/guides/meta-regression-in-meta-analysis) A meta-regression should be considered when clinically important variation in treatment effects is observed graphically or indicated by \\( \\tau^2 \\), not based on chi-squared, Cochrane Q, or I-squared heterogeneity tests.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC8207572/) Significance of a regression weight is commonly tested with a Wald-type z statistic, but the Knapp–Hartung adjustment, which accounts for the uncertainty in estimating \\( \\tau^2 \\) and yields Student's t-based tests, is often advisable because it reduces the risk of false positives.[2](https://doing-meta.guide/metareg) • [5](https://www.stata.com/manuals/metametaregress.pdf) • [12](https://doi.org/10.1002/sim.1482)\n\nModel fit is reported through the pseudo \\( R^2 \\), the percent reduction in heterogeneity variance from the random-effects model to the mixed-effects model.[2](https://doing-meta.guide/metareg) In the classic [BCG vaccine](https://www.edgepedia.org/bcg-vaccine) example with 13 studies, absolute latitude accounted for 75.62% of heterogeneity, reducing \\( \\tau^2 \\) from 0.3132 to 0.0764; at the mean latitude of about 33.5 degrees the estimated average risk ratio was 0.49 (95% CI 0.39 to 0.60).[13](https://www.metafor-project.org/doku.php/tips:computing_adjusted_effects)\n\n## Origin\n\nTheir 1985 book *Statistical Methods for Meta-Analysis* is cited as a foundational reference.[1](https://onlinelibrary.wiley.com/doi/10.1002/jrsm.1338) • [14](https://doi.org/10.2307/2531069) Stanley and Stephen B. Jarrell introduced meta-regression analysis into economics in 1989 in the Journal of Economic Surveys.[1](https://onlinelibrary.wiley.com/doi/10.1002/jrsm.1338) • [15](https://doi.org/10.1111/j.0950-0804.2005.00249.x) The random-effects regression model was published by C. S. Berkey and colleagues in 1995 in [Statistics in Medicine](https://www.edgepedia.org/statistics-in-medicine), illustrated with a meta-analysis of BCG vaccine efficacy against tuberculosis; their paper also marked the first use of the empirical [Bayes estimator](https://www.edgepedia.org/bayes-estimator) in the meta-analytic context.[1](https://onlinelibrary.wiley.com/doi/10.1002/jrsm.1338) • [16](https://doi.org/10.1002/sim.4780140406) • [5](https://www.stata.com/manuals/metametaregress.pdf) Guidance on how meta-regression should be undertaken and interpreted came from [Simon G. Thompson](https://www.edgepedia.org/simon-g-thompson) and Julian P. T. Higgins in 2002 in Statistics in Medicine.[5](https://www.stata.com/manuals/metametaregress.pdf) • [17](https://doi.org/10.1002/sim.1187)\n\n## Variants\n\nSeveral extensions place meta-regression inside larger model classes. van Houwelingen, Arends, and Stijnen's 2002 tutorial presented meta-regression within the general linear mixed model using approximate likelihood, covering univariate and bivariate treatment effects.[18](https://doi.org/10.1002/sim.1040) The mixmeta framework of Francesco Sera and colleagues (2019) defines meta-analysis and meta-regression generally as linear mixed-effects models, subsuming multivariate, network, multilevel, dose-response, and longitudinal meta-analysis and meta-regression as special cases.[19](https://doi.org/10.1002/sim.8362) Multilevel network meta-regression (ML-NMR), published by David M. Phillippo and colleagues in 2020, extends meta-regression to population-adjusted treatment comparisons mixing individual and aggregate data.[20](https://doi.org/10.1111/rssa.12579)\n\nWhen studies contribute multiple, dependent effect sizes, three primary methods model the dependence: multivariate methods, three-level models, and correlated-effects models with robust variance estimation (RVE), the last introduced by Larry V. Hedges, Elizabeth Tipton, and Matthew C. Johnson in 2010; combining three-level models with RVE has been proposed to ensure nominal type I errors.[21](https://www.cambridge.org/core/services/aop-cambridge-core/content/view/22BDF7FF8AAE7AC40E6A6CD32FA7ABD4/S1759287926100921a.pdf/div-class-title-meta-regression-with-categorical-moderators-and-dependent-effect-sizes-a-simulation-study-div.pdf) • [22](https://doi.org/10.1002/jrsm.5) Bayesian random-effects meta-regression is implemented in the bayesmeta R package, which uses a semi-analytical direct algorithm instead of MCMC for fast, reproducible computation.[23](https://ar5iv.labs.arxiv.org/html/2209.06004) RoBMA-reg embeds meta-regression in [Bayesian model averaging](https://www.edgepedia.org/bayesian-model-averaging), jointly accounting for uncertainty in the presence versus absence of the effect, heterogeneity, and publication bias while estimating moderator coefficients.[24](https://fbartos.github.io/RoBMA/articles/v31-robma-metaregression.html) • [25](https://link.springer.com/article/10.1007/s10648-026-10172-1)\n\n## Applications\n\nIn R, the metafor package fits equal-, fixed-, and random-effects models and, by adding study-level moderators, mixed-effects meta-regression models; the rma.uni (alias rma) function provides the general framework via the mods argument, and advanced inference options include the Knapp–Hartung method, permutation tests, and cluster-robust inference.[7](https://wviechtb.github.io/metafor/reference/metafor-package.html) The meta package's metareg function requires only a meta-analysis object and a covariate name; test='knha' applies the Knapp–Hartung adjustment, and method='ML' is recommended when comparing models.[2](https://doing-meta.guide/metareg) Stata's meta regress performs random-effects and fixed-effects meta-regression with \\( \\tau^2 \\) estimators reml, mle, ebayes, dlaird, sjonkman, hedges, or hschmidt.[5](https://www.stata.com/manuals/metametaregress.pdf) The mixmeta package implements the extended mixed-effects framework,[19](https://doi.org/10.1002/sim.8362) and bayesmeta version 3.5 provides the bmr() function for Bayesian meta-regression.[26](https://cran.uib.no/web/packages/bayesmeta/bayesmeta.pdf)\n\nWith a continuous moderator there is no single average effect, so an adjusted effect is computed by plugging a chosen moderator value into the model, analogous to marginal means.[13](https://www.metafor-project.org/doku.php/tips:computing_adjusted_effects)\n\n## Limitations and alternatives\n\nEcological bias is the structural limitation. Treating a study-level association as if it were an individual-level one is the ecological fallacy, also called aggregation bias in this context.[6](https://casrai.org/guides/meta-regression-in-meta-analysis) When a covariate modifies baseline risk but not the treatment effect, pooling over it biases the estimated treatment effect toward the null.[27](https://journals.sagepub.com/doi/10.1177/0272989X13485157) Regressing treatment effect against outcome risk can produce spurious correlations as strong as -0.71 even when the covariate and treatment effect are truly unrelated.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC8207572/) This problem was documented directly by Jesse A. Berlin and colleagues in 2002, comparing individual-patient and group-level meta-regressions of treatment effect modifiers.[28](https://doi.org/10.1002/sim.1023)\n\nBecause patients cannot be randomized to covariate values, meta-regression is inherently observational and inherits confounding, correlation between covariates, and the inability to infer causality from association.[27](https://journals.sagepub.com/doi/10.1177/0272989X13485157) Thompson and Higgins caution that data dredging may be an issue, and overfitting due to small numbers of studies is a recognized danger.[17](https://doi.org/10.1002/sim.1187) • [23](https://ar5iv.labs.arxiv.org/html/2209.06004)\n\nPower is often low. The widely cited rule of thumb is at least ten studies per covariate, so a two-covariate model wants at least twenty studies; Stata's manual cites Borenstein et al. (2021) for the recommendation, while other accounts attribute the rule to Higgins and Thompson's 2004 simulation study showing inflated type I error rates with too few studies.[5](https://www.stata.com/manuals/metametaregress.pdf) • [6](https://casrai.org/guides/meta-regression-in-meta-analysis) Using a cut-off of fewer than 10 studies, 53% (43 of 81) of published meta-regression analyses were at risk of overfitting.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC8207572/) Each covariate costs approximately one degree of freedom, roughly equal to one study.[29](https://link.springer.com/article/10.1186/2046-4053-2-107) The median number of studies in Cochrane meta-analyses (among those with at least two studies) is three, which is problematic because meta-regression requires more.[30](https://doing-meta.guide/power) Statistically insignificant meta-regression results should therefore not be interpreted as evidence that a characteristic does not affect outcomes.[29](https://link.springer.com/article/10.1186/2046-4053-2-107)\n\nIndividual participant data (IPD) meta-analysis allows more powerful and uniformly consistent analyses of subgroups and patient-level interactions than aggregate-data meta-analysis, at greater cost in time and resources; within-trial comparisons avoid ecological bias and have far greater statistical power, making IPD meta-regression the reference standard for individual-level effect modification.[31](https://pmc.ncbi.nlm.nih.gov/articles/PMC7125394/) • [27](https://journals.sagepub.com/doi/10.1177/0272989X13485157)\n\n## References\n\n1. [A history of meta-regression: Technical, conceptual, and practical developments between 1974 and 2018](https://onlinelibrary.wiley.com/doi/10.1002/jrsm.1338)\n2. [Chapter 8 Meta-Regression | Doing Meta-Analysis in R](https://doing-meta.guide/metareg)\n3. [Exploring heterogeneity in meta-analysis using trial-level characteristics (Cochrane Evidence Synthesis and Methods tutorial)](https://www.ovid.com/journals/cesm/fulltext/10.1002/cesm.70099~exploring-heterogeneity-in-meta-analysis-using-trial-level)\n4. [metafor: A Meta-Analysis Package for R (Journal of Statistical Software vignette)](https://cran.r-project.org/web/packages/metafor/vignettes/metafor.pdf)\n5. [Stata [META] meta regress manual](https://www.stata.com/manuals/metametaregress.pdf)\n6. [Meta-Regression in Meta-Analysis: Covariates, the Ecological Fallacy, and the 10-Studies Rule](https://casrai.org/guides/meta-regression-in-meta-analysis)\n7. [metafor: A Meta-Analysis Package for R, metafor-package](https://wviechtb.github.io/metafor/reference/metafor-package.html)\n8. [Fixed-Effects and Random-Effects Models in Meta-Analysis (metafor documentation)](https://search.r-project.org/CRAN/refmans/metafor/html/misc-models.html)\n9. [Larry V. Hedges, Jack L. Vevea (1998). Fixed- and random-effects models in meta-analysis.. Psychological Methods.](https://doi.org/10.1037/1082-989x.3.4.486)\n10. [Meta-analysis in clinical trials (Controlled Clinical Trials, 1986)](https://doi.org/10.1016/0197-2456%2886%2990046-2)\n11. [Most published meta-regression analyses based on aggregate data suffer from methodological pitfalls: a meta-epidemiological study (Geissbühler et al., BMC Medical Research Methodology, 2021)](https://pmc.ncbi.nlm.nih.gov/articles/PMC8207572/)\n12. [Guido Knapp, Joachim Hartung (2003). Improved tests for a random effects meta‐regression with a single covariate. Statistics in Medicine.](https://doi.org/10.1002/sim.1482)\n13. [Computing Adjusted Effects Based on Meta-Regression Models [The metafor Package]](https://www.metafor-project.org/doku.php/tips:computing_adjusted_effects)\n14. [P. R. Freeman, L. V. Hedges, I. Olkin (1986). Statistical Methods for Meta-Analysis.. Biometrics.](https://doi.org/10.2307/2531069)\n15. [T. D. Stanley, Stephen B. Jarrell (1989). META‐REGRESSION ANALYSIS: A QUANTITATIVE METHOD OF LITERATURE SURVEYS. Journal of Economic Surveys.](https://doi.org/10.1111/j.0950-0804.2005.00249.x)\n16. [C. S. Berkey and colleagues (1995). A random‐effects regression model for meta‐analysis. Statistics in Medicine.](https://doi.org/10.1002/sim.4780140406)\n17. [Simon G. Thompson, Julian P. T. Higgins (2002). How should meta‐regression analyses be undertaken and interpreted?. Statistics in Medicine.](https://doi.org/10.1002/sim.1187)\n18. [Hans C. van Houwelingen, Lidia R. Arends, Theo Stijnen (2002). Advanced methods in meta‐analysis: multivariate approach and meta‐regression. Statistics in Medicine.](https://doi.org/10.1002/sim.1040)\n19. [Francesco Sera and colleagues (2019). An extended mixed‐effects framework for meta‐analysis. Statistics in Medicine.](https://doi.org/10.1002/sim.8362)\n20. [David M. Phillippo and colleagues (2020). Multilevel Network Meta-Regression for Population-Adjusted Treatment Comparisons. Journal of the Royal Statistical Society Series A (Statistics in Society).](https://doi.org/10.1111/rssa.12579)\n21. [Meta-regression with categorical moderators and dependent effect sizes: A simulation study (Research Synthesis Methods / Cambridge)](https://www.cambridge.org/core/services/aop-cambridge-core/content/view/22BDF7FF8AAE7AC40E6A6CD32FA7ABD4/S1759287926100921a.pdf/div-class-title-meta-regression-with-categorical-moderators-and-dependent-effect-sizes-a-simulation-study-div.pdf)\n22. [Larry V. Hedges, Elizabeth Tipton, Matthew C. Johnson (2010). Robust variance estimation in meta‐regression with dependent effect size estimates. Research Synthesis Methods.](https://doi.org/10.1002/jrsm.5)\n23. [Using the bayesmeta R package for Bayesian random-effects meta-regression](https://ar5iv.labs.arxiv.org/html/2209.06004)\n24. [Robust Bayesian Model-Averaged Meta-Regression • RoBMA (package vignette)](https://fbartos.github.io/RoBMA/articles/v31-robma-metaregression.html)\n25. [From RAMSing to Rigor: Improving Moderator Analysis in STEM Education Meta-Research (Educational Psychology Review, Springer)](https://link.springer.com/article/10.1007/s10648-026-10172-1)\n26. [bayesmeta: Bayesian Random-Effects Meta-Analysis and Meta-Regression (CRAN, version 3.5, dated 2025-08-29)](https://cran.uib.no/web/packages/bayesmeta/bayesmeta.pdf)\n27. [Evidence Synthesis for Decision Making 3: Heterogeneity, Subgroups, Meta-Regression, Bias, and Bias-Adjustment (Dias et al., Medical Decision Making 2013 / NICE DSU TSD3)](https://journals.sagepub.com/doi/10.1177/0272989X13485157)\n28. [Jesse A. Berlin and colleagues (2002). Individual patient‐ versus group‐level data meta‐regressions for the investigation of treatment effect modifiers: ecological bias rears its ugly head. Statistics in Medicine.](https://doi.org/10.1002/sim.1023)\n29. [Risk of bias: a simulation study of power to detect study-level moderator effects in meta-analysis (Systematic Reviews, 2013)](https://link.springer.com/article/10.1186/2046-4053-2-107)\n30. [Chapter 14 Power Analysis | Doing Meta-Analysis in R](https://doing-meta.guide/power)\n31. [Individual participant data meta-analyses compared with meta-analyses based on aggregate data (Cochrane methodology review)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7125394/)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Mathematics and statistics › Statistics and probability › Statistical inference, estimation, sampling, and testing › Regression analysis*\n\n*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/meta-regression", "markdown_url": "https://www.edgepedia.org/meta-regression.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Meta-regression\", Edgepedia (EdgeChat), https://www.edgepedia.org/meta-regression. Edgepedia Community License 1.0.", "credit_md": "\"[Meta-regression](https://www.edgepedia.org/meta-regression)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/meta-regression](https://www.edgepedia.org/meta-regression). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Meta-regression\", Edgepedia (EdgeChat), https://www.edgepedia.org/meta-regression. Edgepedia Community License 1.0.", "speakable": "Meta-regression is a statistical technique in meta-analysis that regresses the effect sizes of multiple studies on study-level covariates, called moderators, to explain why studies differ." }