Tests of mediation: Paradoxical decline in statistical power as a function of mediator collinearity
Open accessAbstract
Increasing the correlation between the independent variable and the mediator (a coefficient) increases the effect size (ab) for mediation analysis; however, increasing a by definition increases collinearity in mediation models. As a result, the standard error of product tests increase. The variance inflation caused by increases in a at some point outweighs the increase of the effect size (ab) and results in a loss of statistical power. This phenomenon also occurs with nonparametric bootstrapping approaches because the variance of the bootstrap distribution of ab approximates the variance expected from normal theory. Both variances increase dramatically when a exceeds the b coefficient, thus explaining the power decline with increases in a. Implications for statistical analysis and applied researchers are discussed.
The claims and statistics of this paper haven’t been checked yet.
Run the check — free, no account needed. It reads the open-access text and checks the sources it cites.
Integrity signals
Registry
Cites
Statistics
Claims the paper makes
The claims and statistics of this paper haven’t been checked yet.
Run the check — free, no account needed. It reads the open-access text and checks the sources it cites.
Claims other papers make about this one
Each is checked against this paper’s own text: does it say what the citing paper says it does?
A large coefficient relating the independent variable to the mediator increases the error variance of the indirect effect, making it more likely that zero will be contained in bootstrap confidence intervals, especially when the coefficient relating the mediator to the outcome is substantially smaller.
“Moreover, a large coefficient relating the independent variable to the mediator also increases the error variance of the indirect effect, making it more likely that zero will be contained in bootstrap confidence intervals, a problem that is enhanced when the coefficient relating the mediator to the outcome variable is substantially smaller (Beasley, 2014).”
From Responding to uncertain threat: A potential mediator for the effect of mindfulness on anxiety (2021) · cites this paper as Beasley, 2014
Self-check
Percentages, sums and reported differences re-computed from the paper’s own numbers.
The self-check needs the paper’s full text.
Cites
Baron, R. M., & Kenny, D. A. (1986) · Journal of Personality and Social Psychology · doi:10.1037//0022-3514.51.6.1173 · Cited by 91,542 · see citing works
Baron, R. M., & Kenny, D. A. (1986) · Journal of Personality and Social Psychology · doi:10.1037/0022-3514.51.6.1173 · Cited by 91,532 · see citing works
An Introduction to the Bootstrap
Efron, B., & Tibshirani, R. J. (1994) · doi:10.1201/9780429246593 · Cited by 40,500 · see citing works
Preacher, K. J., & Hayes, A. F. (2008) · Behavior Research Methods · doi:10.3758/brm.40.3.879 · Cited by 34,117 · see citing works
An Introduction to the Bootstrap
Efron, B., & Tibshirani, R. J. (1993) · doi:10.1007/978-1-4899-4541-9 · Cited by 21,230 · see citing works
Asymptotic Confidence Intervals for Indirect Effects in Structural Equation Models
Sobel, M. E. (1982) · Sociological Methodology · doi:10.2307/270723 · Cited by 12,810 · see citing works
Mediation in experimental and nonexperimental studies: New procedures and recommendations.
Shrout, P. E., & Bolger, N. (2002) · Psychological Methods · doi:10.1037/1082-989x.7.4.422 · Cited by 11,335 · see citing works
Beyond Baron and Kenny: Statistical Mediation Analysis in the New Millennium
Hayes, A. F. (2009) · Communication Monographs · doi:10.1080/03637750903310360 · Cited by 9,757 · see citing works
A comparison of methods to test mediation and other intervening variable effects
MacKinnon, D. P., Lockwood, C. M., Hoffman, J. M., West, S. G., & Sheets, V. (2002) · Psychological Methods · doi:10.1037/1082-989x.7.1.83 · Cited by 9,406 · see citing works
Confidence Limits for the Indirect Effect: Distribution of the Product and Resampling Methods
MacKinnon, D. P., Lockwood, C. M., & Williams, J. (2004) · Multivariate Behavioral Research · doi:10.1207/s15327906mbr3901_4 · Cited by 7,806 · see citing works
Introduction to Statistical Mediation Analysis
MacKinnon, D. P. (2012) · doi:10.4324/9780203809556 · Cited by 5,618 · see citing works
Multiple regression in behavioral research
Kerlinger, F. N., & Pedhazur, E. J. (1973) · Medical Entomology and Zoology · Cited by 4,877 · see citing works
Required sample size to detect the mediated effect
Fritz, M. S., & MacKinnon, D. P. (2007) · Psychological Science · doi:10.1111/j.1467-9280.2007.01882.x · Cited by 4,810 · see citing works
Multiple Regression in Behavioral Research.
Amick, D. J., Kerlinger, F. N., & Pedhazur, E. J. (1974) · Contemporary Sociology A Journal of Reviews · doi:10.2307/2061986 · Cited by 1,937 · see citing works
Statistical Strategies for Small Sample Research
Hoyle, R. H. (1999) · Cited by 1,896 · see citing works
A Simulation Study of Mediated Effect Measures
MacKinnon, D. P., Warsi, G. M., & Dwyer, J. H. (1995) · Multivariate Behavioral Research · doi:10.1207/s15327906mbr3001_3 · Cited by 1,715 · see citing works
On the Exact Variance of Products
Goodman, L. A. (1960) · Journal of the American Statistical Association · doi:10.1080/01621459.1960.10483369 · Cited by 1,467 · see citing works
Distribution of the product confidence limits for the indirect effect: Program PRODCLIN
MacKinnon, D. P., Fritz, M. S., Williams, J. G., & Lockwood, C. M. (2007) · Behavior Research Methods · doi:10.3758/bf03193007 · Cited by 1,328 · see citing works
Direct and Indirect Effects: Classical and Bootstrap Estimates of Variability
Bollen, K. A., & Stine, R. (1990) · Sociological Methodology · doi:10.2307/271084 · Cited by 1,305 · see citing works
Resampling and Distribution of the Product Methods for Testing Indirect Effects in Complex Models
Williams, J., & MacKinnon, D. P. (2008) · Structural Equation Modeling: A Multidisciplinary Journal · doi:10.1080/10705510701758166 · Cited by 1,240 · see citing works
Chen, H.-T. (1990) · Medical Entomology and Zoology · Cited by 1,227 · see citing works
Intermediate phenotypes and genetic mechanisms of psychiatric disorders
Meyer-Lindenberg, A., & Weinberger, D. R. (2006) · Nature reviews. Neuroscience · doi:10.1038/nrn1993 · Cited by 1,216 · see citing works
On a distinction between hypothetical constructs and intervening variables.
Maccorquodale, K., & Meehl, P. (1948) · Psychology Review · doi:10.1037/h0056029 · Cited by 1,035 · see citing works
Advances in testing the statistical significance of mediation effects.
Mallinckrodt, B. S., Abraham, W. T., Wei, M., & Russell, D. W. (2006) · Journal of Counseling Psychology · doi:10.1037/0022-0167.53.3.372 · Cited by 1,010 · see citing works
A General Model for Testing Mediation and Moderation Effects
Fairchild, A. J., & MacKinnon, D. P. (2009) · Prevention Science · doi:10.1007/s11121-008-0109-6 · Cited by 918 · see citing works
Lipsey, M. W. (1991) · Evaluation and Program Planning · doi:10.1016/0149-7189(91)90027-e · Cited by 592 · see citing works
The Probability Function of the Product of Two Normally Distributed Variables
Aroian, L. A. (1947) · The Annals of Mathematical Statistics · doi:10.1214/aoms/1177730442 · Cited by 535 · see citing works
Assessing Mediational Models: Testing and Interval Estimation for Indirect Effects
Biesanz, J. C., Falk, C. F., & Savalei, V. (2010) · Multivariate Behavioral Research · doi:10.1080/00273171.2010.498292 · Cited by 397 · see citing works
Explanation of Two Anomalous Results in Statistical Mediation Analysis
Fritz, M. S., Taylor, A. B., & MacKinnon, D. P. (2012) · Multivariate Behavioral Research · doi:10.1080/00273171.2012.640596 · Cited by 358 · see citing works
A mixture model approach for the analysis of microarray gene expression data
Allison, D. B., Gadbury, G. L., Heo, M., Fernández, J. R., Lee, C., Prolla, T. A., & Weindruch, R. H. (2002) · Computational Statistics & Data Analysis · doi:10.1016/s0167-9473(01)00046-9 · Cited by 358 · see citing works
Comparison of methods for constructing confidence intervals of standardized indirect effects
Cheung, M. W. ‐. (2009) · Behavior Research Methods · doi:10.3758/brm.41.2.425 · Cited by 279 · see citing works
The Distribution of Products of Independent Random Variables
Springer, M. D., & Thompson, W. E. (1966) · SIAM Journal on Applied Mathematics · doi:10.1137/0114046 · Cited by 272 · see citing works
Cheung, M. W. ‐. (2007) · Structural Equation Modeling A Multidisciplinary Journal · doi:10.1080/10705510709336745 · Cited by 258 · see citing works
Stoltenberg, S. F., Twitchell, G. R., Hanna, G. L., Cook, E. H., Fitzgerald, H. E., Zucker, R. A., & Little, K. Y. (2002) · American Journal of Medical Genetics · doi:10.1002/ajmg.10187 · Cited by 143 · see citing works
Mediation analysis via potential outcomes models
Albert, J. M. (2007) · Statistics in Medicine · doi:10.1002/sim.3016 · Cited by 138 · see citing works
A Comparison of Single Sample and Bootstrap Methods to Assess Mediation in Cluster Randomized Trials
Pituch, K. A., Stapleton, L. M., & Kang, J. Y. (2006) · Multivariate Behavioral Research · doi:10.1207/s15327906mbr4103_5 · Cited by 135 · see citing works
On the Distribution of Products of Random Variables
Łomnicki, Z. A. (1967) · Journal of the Royal Statistical Society Series B (Statistical Methodology) · doi:10.1111/j.2517-6161.1967.tb00713.x · Cited by 110 · see citing works
Barger, J. L., Kayo, T., Pugh, T. D., Prolla, T. A., & Weindruch, R. H. (2008) · Experimental Gerontology · doi:10.1016/j.exger.2008.06.013 · Cited by 89 · see citing works
Explaining the Relationship between Three Eye Diseases and Depressive Symptoms in Older Adults
Popescu, M. L., Boisjoly, H., Schmaltz, H. N., Kergoat, M., Rousseau, J., Moghadaszadeh, S., Djafari, F., & Freeman, E. E. (2012) · Investigative Ophthalmology & Visual Science · doi:10.1167/iovs.11-9330 · Cited by 85 · see citing works
Allison, D. B., Heo, M., Schork, N. J., Wong, S.-L., & Elston, R. C. (1998) · Human Heredity · doi:10.1159/000022788 · Cited by 69 · see citing works
Meulders, A., Vansteenwegen, D., & Vlaeyen, J. W. S. (2012) · Pain · doi:10.1016/j.pain.2012.02.005 · Cited by 67 · see citing works
Forestell, C., & Mennella, J. (2012) · Appetite · doi:10.1016/j.appet.2012.03.005 · Cited by 62 · see citing works
Candidate gene studies in the 21st century: meta‐analysis, mediation, moderation
Munafo, M. R. (2006) · Genes Brain & Behavior · doi:10.1111/j.1601-183x.2006.00188.x · Cited by 55 · see citing works
Understanding Causes and Generalizing About Them
SECHREST, L. B., & Scott, A. G. (1993) · Medical Entomology and Zoology · Cited by 45 · see citing works
The neighborhood social environment and body mass index among youth: a mediation analysis
Veitch, J., van Stralen, M. M., Chinapaw, M. J. M., Velde, S. J. T., Crawford, D. A., Salmon, J., & Timperio, A. F. (2012) · International Journal of Behavioral Nutrition and Physical Activity · doi:10.1186/1479-5868-9-31 · Cited by 42 · see citing works
Comparing Groups: Randomization and Bootstrap Methods Using R
Zieffler, A., Harring, J. R., & Long, J. D. (2011) · Cited by 37 · see citing works
Cox, T. L., Ard, J. D., Beasley, T. M., Fernández, J. R., Howard, V. J., & Affuso, O. (2011) · Journal of Women s Health · doi:10.1089/jwh.2010.2637 · Cited by 37 · see citing works
Self-citationZieffler, A., Harring, J. R., & Long, J. D. (2011) · Wiley eBooks · doi:10.1002/9781118063682 · Cited by 33 · see citing works
Anomalous bodily-self experiences among recreational ketamine users
Wilkins, L. K., Girard, T. A., & Cheyne, J. A. (2012) · Cognitive Neuropsychiatry · doi:10.1080/13546805.2012.663162 · Cited by 17 · see citing works
CAI as an Adjunct to Teaching Introductory Statistics: Affect Mediates Learning
Varnhagen, C. K., & Zumbo, B. D. (1990) · Journal of Educational Computing Research · doi:10.2190/llt0-efta-6tv8-4w82 · Cited by 14 · see citing works
Hanks, L. J., Casazza, K., Willig, A., Cardel, M., Beasley, T., & Fernández, J. R. (2010) · Jornal de Pediatria · doi:10.1016/j.jpeds.2010.02.065 · Cited by 8 · see citing works
Self-citationHeterogeneity of variance and biased hypothesis tests
Zimmerman, D. W. (2012) · Journal of Applied Statistics · doi:10.1080/02664763.2012.740620 · Cited by 5 · see citing works
Power of Product Tests of Mediation as a Function of Mediator Collinearity
Beasley, T. M. (2012) · General Linear Model Journal · Cited by 4 · see citing works
Self-citation
Flagged first, then by citations
Research works printed but not linked: 1
- Cochran WG Sampling Techniques 1963 Wiley New York
Cited by
Sorted by citations
The role of psychosocial processes in the development and maintenance of chronic pain
Edwards, R. R., Dworkin, R. H., Sullivan, M. D., Turk, D. C., & Wasan, A. D. (2016) · The Journal of Pain · doi:10.1016/j.jpain.2016.01.001 · Cited by 954 · see citing works
A novel measure of effect size for mediation analysis
Lachowicz, M. J., Preacher, K. J., & Kelley, K. (2018) · Psychological Methods · doi:10.1037/met0000165 · Cited by 271 · see citing works
A cautionary note on the power of the test for the indirect effect in mediation analysis
Loeys, T., Moerkerke, B., & Vansteelandt, S. (2015) · Frontiers in Psychology · doi:10.3389/fpsyg.2014.01549 · Cited by 110 · see citing works
Rothenberg, K. A., Stern, J. R., George, E., Trickey, A. W., Morris, A. M., Hall, D. E., Johanning, J. M., Hawn, M. T., & Arya, S. (2019) · JAMA Network Open · doi:10.1001/jamanetworkopen.2019.4330 · Cited by 104 · see citing works
Gratefulness and subjective well-being: Social connectedness and presence of meaning as mediators.
Liao, K. Y., & Weng, C. (2018) · Journal of Counseling Psychology · doi:10.1037/cou0000271 · Cited by 84 · see citing works
Geijselaers, S. L. C., Sep, S. J. S., Claessens, D., Schram, M. T., van Boxtel, M. P. J., Henry, R. M. A., Verhey, F. R. J., Kroon, A. A., Dagnelie, P. C., Schalkwijk, C. G., et al. (2017) · Diabetes Care · doi:10.2337/dc17-0330 · Cited by 75 · see citing works
Responding to uncertain threat: A potential mediator for the effect of mindfulness on anxiety
Papenfuss, I., Lommen, M. J. J., Grillon, C., Balderston, N. L., & Ostafin, B. D. (2021) · Journal of Anxiety Disorders · doi:10.1016/j.janxdis.2020.102332 · Cited by 38 · see citing works
“8 outcome variable is substantially smaller (Beasley, 2014).”
BackgroundMulticollinearity: An Overview and Introduction of Ridge PLS-SEM Estimation
Streukens, S., & Leroi‐Werelds, S. (2023) · Springer eBooks · doi:10.1007/978-3-031-37772-3_7 · Cited by 36 · see citing works
Experimental Power for Indirect Effects in Group-randomized Studies with Group-level Mediators
Kelcey, B., Dong, N., Spybrook, J. K., & Shen, Z. (2017) · Multivariate Behavioral Research · doi:10.1080/00273171.2017.1356212 · Cited by 32 · see citing works
Türk, F., Kellett, S., & Waller, G. (2021) · Body Image · doi:10.1016/j.bodyim.2021.01.007 · Cited by 27 · see citing works
Indirect effects in mediation analyses should not be tested for statistical significance
Twisk, J. W. R. (2024) · Journal of Clinical Epidemiology · doi:10.1016/j.jclinepi.2024.111393 · Cited by 21 · see citing works
Hong, Y., Chen, M., Chen, C., & Qiu, M. (2024) · BMC Nursing · doi:10.1186/s12912-024-02025-5 · Cited by 18 · see citing works
Liao, Y., Xuelan, L., Wu, X., Li, C., & Li, Y. (2024) · BMC Psychology · doi:10.1186/s40359-024-02304-5 · Cited by 19 · see citing works
Job burnout and anxiety among medical staff: A latent profile and moderated mediation analysis
Hao, S., & Zhang, X. (2024) · Social Science & Medicine · doi:10.1016/j.socscimed.2024.117141 · Cited by 14 · see citing works
Jiang, Y., Hou, W., Gao, J., & Wu, S. (2022) · The Science of The Total Environment · doi:10.1016/j.scitotenv.2022.161187 · Cited by 13 · see citing works
Cui, X.-J., Xie, B., Zhu, K., Liao, Q.-Q., Zhou, J.-C., Du, S., Liu, X.-X., Chen, Z.-J., Yang, Y. H., & Yi, X. (2024) · Scientific Reports · doi:10.1038/s41598-024-80727-1 · Cited by 12 · see citing works
Kelcey, B., Xie, Y., Spybrook, J. K., & Dong, N. (2020) · Multivariate Behavioral Research · doi:10.1080/00273171.2020.1738910 · Cited by 12 · see citing works
Kelcey, B., Spybrook, J. K., Dong, N., & Bai, F. (2020) · Journal of Research on Educational Effectiveness · doi:10.1080/19345747.2020.1726540 · Cited by 12 · see citing works
Strengths use and thriving at work among nurses: a latent profile and mediation analysis
Wu, J., Shen, Z., Ouyang, Z., Xiang, Y., Ding, R., Liao, Y., & Chen, L. (2025) · BMC Nursing · doi:10.1186/s12912-025-02715-8 · Cited by 13 · see citing works
Peterson, J. A., Lohman, C., Larson, R. D., Bemben, M. G., & Black, C. D. (2022) · Journal of Pain Research · doi:10.2147/jpr.s387635 · Cited by 8 · see citing works
Statistical power in partially nested designs probing multilevel mediation
Kelcey, B., Bai, F., & Xie, Y. (2020) · Psychotherapy Research · doi:10.1080/10503307.2020.1717012 · Cited by 7 · see citing works
Leavy, B., O’Connell, B. H., & O’Shea, D. E. (2023) · Biological Psychology · doi:10.1016/j.biopsycho.2023.108663 · Cited by 7 · see citing works
Power of Mediation Effects Using Bootstrap ResamplingPreprint
Steffener, J. (2021) · arXiv (Cornell University) · doi:10.48550/arxiv.2106.02482 · Cited by 7 · see citing works
Liao, Y., Wei, W., Fang, S., Wu, L., Gao, J., Wu, X., Huang, L., Li, C., & Li, Y. (2023) · BMC Nursing · doi:10.1186/s12912-023-01467-7 · Cited by 6 · see citing works
Strategies for Efficient Experimental Design in Studies Probing 2-1-1 Mediation
Kelcey, B., & Shen, Z. (2019) · The Journal of Experimental Education · doi:10.1080/00220973.2018.1533796 · Cited by 6 · see citing works
Liao, Y., Huang, M., Gu, Z., Li, C., Yu, Y., Zhang, Q., Lai, X., Liu, J., He, K., Chu, H., et al. (2025) · BMC Nursing · doi:10.1186/s12912-025-03754-x · Cited by 4 · see citing works
Manns, L., Vögele, C., Stammet, P., & Schulz, A. (2025) · Journal of Occupational Medicine and Toxicology · doi:10.1186/s12995-025-00461-w · Cited by 4 · see citing works
Wang, F., & Ma, X. (2025) · BMC Psychiatry · doi:10.1186/s12888-025-07179-3 · Cited by 5 · see citing works
University of Alabama at Birmingham Nathan Shock Center: comparative energetics of aging
Austad, S. N., Buford, T. W., Allison, D. B., Ballinger, S. W., Brown, A. W., Carter, C. S., Darley-Usmar, V. M., Hartman, J. L., Nagy, T. R., Smith, D. L., et al. (2021) · GeroScience · doi:10.1007/s11357-021-00414-1 · Cited by 3 · see citing works
Eitze, S., Sprengholz, P., Korn, L., Shamsrizi, P., Felgendreff, L., & Betsch, C. (2023) · Vaccine X · doi:10.1016/j.jvacx.2023.100417 · Cited by 4 · see citing works
Huang, Y. L., Kang, A., Gao, D., Li, J. H., Zhang, H., Yan, M., Gan, X., & Zhou, B. F. (2025) · Environmental Management · doi:10.1007/s00267-025-02171-9 · Cited by 3 · see citing works
Zaid, A., & Adnan, A. (2019) · UTHM Institutional Repository (Universiti Tun Hussein Onn Malaysia) · Cited by 3 · see citing works
Zhou, J., Huang, X., Yuan, D., Wang, L., Liu, M., Jiang, X., Xing, L., & Song, X. (2025) · PeerJ · doi:10.7717/peerj.20485 · Cited by 1 · see citing works
Zhao, W., Zhang, W., Ma, S., Zhang, Y., Nan, Y., Li, X., Duan, C., Gao, S., Zhou, Y., & Zhang, Y. (2026) · Healthcare · doi:10.3390/healthcare14050559 · Cited by 1 · see citing works
Saugui, A. O. (2017) · UTHM Institutional Repository (Universiti Tun Hussein Onn Malaysia)
Kaloutsa, K., Tsinaslanidis, P. E., & Laspita, S. (2025) · Springer proceedings in business and economics · doi:10.1007/978-3-031-76654-1_26
Bao, R., Wade, L., Leahy, A. A., Beauchamp, M. R., Diallo, T. M. O., Smith, J. J., Hillman, C. H., & Lubans, D. R. (2025) · Medicine & Science in Sports & Exercise · doi:10.1249/mss.0000000000003928
Li, Y., Huang, J., Kong, L., & Lyu, Y. (2026) · Frontiers in Psychiatry · doi:10.3389/fpsyt.2026.1734868
Gao, M., Ou, J., Cao, H. G., Suo, D., Tang, Z., & Zhang, N. (2026) · Journal of Health Psychology · doi:10.1177/13591053261443097
- Authors
- Beasley, T. M.
- Affiliations
- United States Department of Veterans Affairs (US)
- Corresponding author
- Beasley, T. Mark
- ORCID
- Beasley, T. Mark
- Venue
- The Journal of Experimental Education
- Publisher
- Informa UK Limited
- Published
- 2013-08-12
- Volume
- 82
- Issue
- 3
- Pages
- 283-306
- ISSN
- 0022-0973 (ISSN-L)
- 1940-0683
- PMC
- PMC4061717
- OpenAlex
- W2152230819
- Cited by
- 39
- References
- 52
- Citations by year
- 2026: 3 · 2025: 8 · 2024: 5 · 2023: 4 · 2022: 2 · 2021: 3 · 2020: 4 · 2019: 3 · 2018: 1 · 2017: 4 · 2016: 1 · 2015: 1
- FWCI
- 0.59
- Type
- journal article
- Language
- en
- Open access
- green
- www.ncbi.nlm.nih.gov
- Locations
- The Journal of Experimental Education (publishedVersion)
- PubMed (publishedVersion)
- PubMed Central (acceptedVersion)
- Funders
- National Institute of Arthritis and Musculoskeletal and Skin Diseases (P60AR048095)
- Topics
- Physical Sciences › Mathematics › Statistics and Probability › Advanced Causal Inference Techniques
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods and Bayesian Inference
- Social Sciences › Decision Sciences › Management Science and Operations Research › Psychometric Methodologies and Testing
- Keywords
- mediation analysis, statistical power, variance inflation, effect size, nonparametric bootstrap
About these checks
Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.
Statistics and arithmetic are recomputed, not judged.
Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.
What counts as a claim ›How we measure confidence ›How we check this ›
Are you an author, or do you see a problem with this page? Request a review
