What Is Analysis of Covariance (ANCOVA) and How to Correct- Ly Report Its Results in Medical Research?

What Is Analysis of Covariance (ANCOVA) and How to Correct- Ly Report Its Results in Medical Research?

Iran J Public Health, Vol. 49, No.5, May 2020, pp.1016-1017 Letter to the Editor What Is Analysis of Covariance (ANCOVA) and How to Correct- ly Report its Results in Medical Research? Alireza KHAMMAR 1, Mohammad YARAHMADI 2, *Farzan MADADIZADEH 3,4 1. Department of Occupational Health Engineering, School of Health, Zabol University of Medical Sciences, Zabol, Iran 2. Razi Herbal Medicines Research Center, Lorestan University of Medical Sciences, Khoramabad, Iran 3. Noncommunicable Diseases Research Center, Fasa University of Medical Sciences, Fasa, Iran 4. Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran *Corresponding Author: Email: [email protected] (Received 09 Dec 2018; accepted 19 Dec 2018) Dear Editor-in-Chief Sometimes in medical researches, a variable that groups. The prior familiarity of some students is not among the main research variables may with the topics leads to increased learning scores. affect the dependent variable and its relation with Therefore, it is a covariate variable. Another ex- the independent variable, which, if identified, can ample of covariate variable is a pretest score in an be involved in the modeling and its linear effect interventional study that needs to identify, meas- are controlled. This is not a dependent or inde- ure and control before the intervention. pendent variable, this type of variable is known as For more information on the use of the AN- covariate (1). COVA methodology and the appropriate way of To control the effect of covariate variable, not reporting the results, note the following points: only the changes in variance of the dependent The dependent variable must be a con- variable are examined (ANOVA), but also the tinuous quantitative variable and have a relationship between the dependent variable and normal distribution. covariate in different levels of a qualitative varia- The covariate must be a continuous ble is analyzed (Regression) (2). The statistical quantitative variable (2). method that can combine ANOVA and The levels of the qualitative variable must Regression for adjusting linear effect of covariate be independent (2). and make a clearer picture is called the analysis of There should be a linear relationship be- covariance (ANCOVA) (1). tween the dependent variable and covari- ANCOVA discovers the variance changes of the ate (3). If the relationship was non-linear, dependent variable due to change in covariate the Multivariate ANOVA method would variable and discriminates it from the variance be useful by considering the covariate as a changes due to changes in the levels of the quali- secondary dependent variable another tative variable; so it reduces the uncertain chang- way is using linear transformations and es of the variance of dependent variable (error) applying ANCOVA to converted varia- and make pure results as well as increases the an- bles (3). alytical power (3). For example, examining the rate of learning the The sign (+ or -) and size of the correla- medical lessons in the students of different tion coefficient between the dependent variable and covariate should be the same 1016 Available at: http://ijph.tums.ac.ir Khammar et al.: What Is Analysis of Covariance (ANCOVA) and How to … at each level of the qualitative variable (1). Hopefully, by considering the above notes, not In other words, if we draw a regression only researchers become more familiar with the line for the relationship between the de- ANCOVA method, but also the medical field pendent variable and covariate at each studies are further enhanced by providing the level of the qualitative variable, the slope appropriate results of statistical methods. of the regression lines should be the same at all levels (Homogeneity of regression Conflict of interest slopes) (2). The independent variable has no relation- The authors declare that there is no conflict of ship with the covariate variable and that it interest. does not affect the relationship between the dependent variable and covariate References (4,5). How to correctly report ANCOVA results: 1. Karpen SC (2017). Misuses of Regression and Reporting the correlation coefficient and ANCOVA in Educational Research. Am J significant P-value of investigating the re- Pharm Educ, 81(8):6501. lationship between the dependent varia- 2. Field A (2013). Discovering statistics using IBM SPSS ble and covariate (4). statistics. 4th ed. Sage, London, pp. :486-89. 3. Rutherford A (2001). Introducing ANOVA and Reporting insignificance relationship be- ANCOVA: a GLM approach. 2nd ed. Sage, tween covariate and independent variable London, pp.: 105. and thus the equality of slope of regres- 4. Schneider BA, Avivi-Reich M, Mozuraitis M sion lines. (2015). A cautionary note on the use of the Provision summary table of the means of Analysis of Covariance (ANCOVA) in classi- dependent variable before and after the fication designs with and without within- adjustment the effect of covariate with subject factors. Front Psychol, 6:474. separately reporting the p-value of means 5. Franzen M (2017). ANCOVA/MANCOVA. st comparison. Encyclopedia of Clinical Neuropsychology. 1 ed. Springer, New York, pp.:160-161. ANCOVA is a type of ANOVA with controlling linear effect of covariate variable by using regres- sion analysis. Available at: http://ijph.tums.ac.ir 1017 .

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