Statistical significance means that a result may not be the cause of random variations within the data. But not every significant result refers to an effect with a high impact, resp. it may even describe a phenomenon that is not really perceivable in everyday life. Statistical significance mainly depends on the sample size, the quality of the data and the power of the statistical procedures. If large data sets are at hand, as it is often the case f. e. in epidemiological studies or in large scale assessments, very small effects may reach statistical significance. In order to describe, if effects have a relevant magnitude, effect sizes are used to describe the strength of a phenomenon. The most popular effect size measure surely is Cohen's d (Cohen, 1988).
Here you will find a number of online calculators for the computation of different effect sizes and an interpretation table at the bottom of this page:
If the two groups have the same n, then the effect size is simply calculated by subtracting the means and dividing the result by the pooled standard deviation. The resulting effect size is called d_{Cohen} and it represents the difference between the groups in terms of their common standard deviation. It is used f. e. for calculating the effect for prepost comparisons in single groups.
In case of relevant differences in the standard deviations, Glass suggests not to use the pooled standard deviation but the standard deviation of the control group. He argues that the standard deviation of the control group should not be influenced, at least in case of nontreatment control groups. This effect size measure is called Glass' Δ ("Glass' Delta"). Please type the data of the control group in column 1 for the correct calculation of Glass' Δ.
Group 1  Group 2  
Mean  
Standard Deviation  
Effect Size d_{Cohen}  
Effect Size Glass' Δ 
N (Total number of observations in both groups) 

Confidence Coefficient  
Confidence Interval for d_{Cohen} 
Analogously, the effect size can be computed for groups with different sample size, by adjusting the calculation of the pooled standard deviation with weights for the sample sizes. This approach is overall identical with d_{Cohen} with a correction of a positive bias in the pooled standard deviation. In the literature, usually this computation is called Cohen's d as well. Please have a look at the remarks bellow the table.
Additionally, you can compute the confidence interval for the effect size and chose a desired confidence coefficient (calculation according to Hedges & Olkin, 1985, p. 86).
Group 1  Group 2  
Mean  
Standard Deviation  
Sample Size (N)  
Effect Size d_{Cohen}, g_{Hedges} ^{*} 
Confidence Coefficient  
Confidence Interval 
Intervention studies usually compare the development of at least two groups (in general an experimental group and a control group). In many cases, the pretest means and standard deviations of both groups do not match and there are a number of possibilities to deal with that problem. Klauer (2001) proposes to compute g_{} for both groups and to substract them afterwards. This way, different sample sizes and pre test values are automatically corrected. The calculation is therefore equal to computating the effect sizes of both groups via form 2 and afterwards to substract both. Morris (2008) presents different effect sizes for repeated measures designs and does a simulation study. He argues to use the pooled pretest standard deviation for weighting the differences of the prepostmeans (so called d_{ppc2} according to Carlson & Smith, 1999). That way, the intervention does not influence the standard deviation. Additionally, there are weighting to correct for the estimation of the population effect size. Usually, Klauer (2001) and Morris (2008) yield similar results.
The downside to this approach: The preposttests are not treated as repeated measures but as independent data. For dependent tests, you can use calculator 4 or 5 or transform eta square from repeated measures in order to account for dependences between measurement points.
Intervention Group  Control Group  
Pre  Post  Pre  Post  
Mean  
Standard Deviation  
Sample Size (N)  
Effect Size d_{ppc2} sensu Morris (2008)  
Effect Size d_{Korr} sensu Klauer (2001) 
While steps 1 to 3 target at comparing independent groups, especially in intervention research, the results are usually based on intraindividual changes in test scores. Morris & DeShon (2008, p.111) suggest a procedure to estimate the effect size for singlegroup pretestposttest designs by taking the correlation between the pre and posttest into account:
In case, the correlation is .5, the resulting effect size equals the independent groups effect size. Higher values lead to an increase in the effect size. Morris & DeShon (2008) suggest to use the standard deviation of the pre test, as this value is not influenced by the intervention. The following calculator both reports the acording effect size and as well reports the effect size based on the pooled standard deviation:
Group 1  Group 2  
Mean  
Standard Deviation  
Correlation  
Effect Size d_{Repeated Measures}  
Effect Size d_{Repeated Measures, pooled} 
N  
Confidence Coefficient  
Confidence Interval for d_{RM} 
Thanks to Sven van As for pointing us to this effect size.
Effect sizes can be obtained by using the tests statistics from hypothesis tests, like Student t tests, as well. In case of independent samples, the result is essentially the same as in effect size calculation #2.
Dependent testing usually yields a higher power, because the interconnection between data points of different measurements are kept. This may be relevant f. e. when testing the same persons repeatedly, or when analyzing test results from matched persons or twins. Accordingly, more information may be used when computing effect sizes. Please note, that this approach largely has the same results compared to using a ttest statistic on gain scores and using the independent sample approach (Morris & DeShon, 2002, p. 119).
Please choose the mode of testing (dependent vs. independent) and specify the t statistic. In case of a dependent t test, please type in the number of cases and the correlation between the two variables. In case of independent samples, please specify the number of cases in each group. The calculation is based on the formulas reported by Borenstein (2009, pp. 228).
Mode of testing  
Student t Value  
n_{1}  
n_{2}  
r  
Effect Size d 
^{*} Wie used the formula t_{c} described in Dunlop, Cortina, Vaslow & Burke (1996, S. 171) in order to calculate d from dependend ttests. Simulations proved it to have the least distortion in estimating d. We would like to thank Frank Aufhammer for pointing us to this publication. In case, the correlation is unknown, please fill in 0. The results will be a conservative estimation in this case, because standard errors will not be controlled then.
A very easy to interpret effect size from analyses of variance (ANOVAs) is η^{2} that reflects the explained proportion variance of the total variance. This proportion may be transformed directly into d. If η^{2} is not available, the F value of the ANOVA can be used as well, as long as the sample size is known. The following computation only works for ANOVAs with two distinct groups (df1 = 1; Thalheimer & Cook, 2002):
FValue  
Sample Size of the Treatment Group  
Sample Size of the Controll Group  
Effect Size d 
In case, the groups means are known from ANOVAs with multiple groups, it is possible to compute the effect sizes f and d (Cohen, 1988, S. 273 ff.). Prior to computing the effect size, you have to determine the minimum and maximum mean and to calculate the between groups standard deviation σ_{m} manually:
Additionally, you have to decide, which scenario fits the data best:
Highest Mean (m_{max})  
Lowest Mean (m_{min})  
Between Group Std (σ_{m})  
Std (σ for the complete sample)  
Number of Groups  
Distribution of Means  
Effect Size f  
Effect Size d 
Measures of effect size like d or correlations can be hard to communicate, e. g. to patients. If you use r^{2} f. e., effects seem to be really small and when a person does not know or understand the interpretation guidelines, even effective interventions could be seen as futile. And even small effects can be very important, as Hattie (2007) underlines:
Rosenthal and Rubin (1982) suggest another way of looking on the effects of treatments by considering the increase of success through interventions. The approach is suitable for 2x2 contingency tables with the different treatment groups in the rows and the number of cases in the columns. The BESD is computed by subtracting the probability of success from the intervention an the control group. The resulting percentage can be transformed into d_{Cohen}.
Another measure, that is widely used in evidence based medicine, is the so called Number Needed to Treat. It shows, how many people are needed in the treatment group in order to obtain at least one additional favorable outcome. In case of a negative value, it is called Number Needed to Harm.
Please fill in the number of cases with a fortunate and unfortunate outcome in the different cells:
Success  Failure  Probability of Success  
Intervention group  
Control Group  
Binomial Effect Size Display (BESD) (Increase of Intervention Success) 

Number Needed to Treat  
r_{Phi}  
Effect Size d_{cohen}  
A conversion between NNT and other effect size measures liken Cohen's d is not easily possible. Concerning the example above, the transformation is done via the pointbiserial correlation r_{phi} which is nothing but an estimation. It leads to a constant NNT indepentent from the sample size and this is in line with publications like Kraemer and Kupfer (2006). Alternative approaches (comp. Furukawa & Leucht, 2011) allow to convert between d and NNT with a higher precision and usually they lead to higher numbers. The Kraemer et al. (2006) approach therefore seems to probably overestimate the effect and it seems to yield accurate results essentially, when normal distribution of the raw values is given. Please have a look at the Furukawa and Leucht (2011) paper for further information:
Cohen's d  Number Needed to Treat (NNT) 
Studies, investigating if specific incidences occur (e. g. death, healing, academic success ...) on a binary basis (yes versus no), and if two groups differ in respect to these incidences, usually Odds Ratios, Risk Ratios and Risk Differences are used to quantify the differences between the groups (Borenstein et al. 2009, chap. 5). These forms of effect size are therefore commonly used in clinical research and in epidemiological studies:
Incidence  no Incidence  N  
Teatment  
Control  
 
Risk Ratio  Odds Ratio  Risk Difference  
Result  
Log  
Estimated Variance V  
Estimated Standard Error SE  
Yule's Q 
Cohen (1988, S. 109) suggests an effect size measure with the denomination q that permits to interpret the difference between two correlations. The two correlations are transformed with Fisher's Z and subtracted afterwards. Cohen proposes the following categories for the interpretation: <.1: no effect; .1 to .3: small effect; .3 to .5: intermediate effect; >.5: large effect.
Correlation r_{1}  
Correlation r_{2}  
Cohen's q  
Interpretation 
Especially in meta analytic research, it is often necessary to average correlations or to perform significance tests on the difference between correlations. Please have a look at our page Testing the Significance of Correlations for online calculators on these subjects.
Most statistical procedures like the computation of Cohen's d or eta;^{2} at least interval scale and distribution assumptions are necessary. In case of categorial or ordinal data, often nonparametric approaches are used  in the case of statistical tests for example Wilcoxon or MannWhitneyU. The distributions of the their test statistics are approximated by normal distributions and finally, the result is used to assess significance. Accordingly, the test statistics can be transformed in effect sizes (comp. Fritz, Morris & Richler, 2012, p. 12; Cohen, 2008). Here you can find an effect size calculator for the test statistics of the Wilcoxon signedrank test, MannWhitneyU or KruskalWallisH in order to calculate η^{2}. You alternatively can directly use the resulting z value as well:
Test  
Teststatistik ^{*} 

n_{2} 

n_{2} 

Eta squared (η^{2})  
d_{Cohen}^{**} 
^{*} Note: Please do not use the sum of the ranks but instead directly type in the test statistics U, W or z from the inferential tests. As Wilcoxon relies on dependent data, you only need to fill in the total sample size. For KruskalWallis please as well specify the total sample size and the number of groups. For z, please fill in the total number of observations (either the total sample size in case of independent tests or for dependent measures with single groups the number of individuals multiplied with the number of assessments; many thanks to Helen AskellWilliams for pointing us this aspect).
^{**} Transformation of η^{2} is done with the formulas of Transformation of the effect sizes d, r, f, Odds Ratio and ?2.
In order to compute Conhen's d, it is necessary to determine the mean (pooled) standard deviation. Here, you will find a small tool that does this for you. Different sample sizes are corrected as well:
Group 1  Group 2  
Standard Deviation  
Sample size (N)  
Pooled Standard Deviation s_{pool} 
Please choose the effect size, you want to transform, in the dropdown menu. Specify the magnitude of the effect size in the text field on the right side of the dropdown menu afterwards. The transformation is done according to Cohen (1988), Rosenthal (1994, S. 239) and Borenstein, Hedges, Higgins, und Rothstein (2009; transformation of d in Odds Ratios).
Effect Size  
d  
r  
η^{2}  
f  
Odds Ratio  
Number Needed to Treat (NNT) 
The χ^{2} and z test statistics from hypothesis tests can be used to compute d and r(Rosenthal & DiMatteo, 2001, p. 71; comp. Elis, 2010, S. 28). The calculation is however only correct for χ^{2} tests with one degree of freedom. Please choose the tests static measure from the dropdown menu und specify the value and N. The transformation from d to r and η^{2} is based on the formulas used in the prior section.
Test Statistic  
N  
d  
r  
η^{2} 
Here, you can see the suggestions of Cohen (1988) and Hattie (2009 S. 97) for interpreting the magnitude of effect sizes. Hattie refers to real educational contexts and therefore uses a more benignant classification, compared to Cohen. We slightly adjusted the intervals, in case, the interpretation did not exactly match the categories of the original authors.
d  r^{*}  η^{2}  Interpretation sensu Cohen (1988)  Interpretation sensu Hattie (2007) 
< 0  < 0    Adverse Effect  
0.0  .00  .000  No Effect  Developmental effects 
0.1  .05  .003  
0.2  .10  .010  Small Effect  Teacher effects 
0.3  .15  .022  
0.4  .2  .039  Zone of desired effects  
0.5  .24  .060  Intermediate Effect  
0.6  .29  .083  
0.7  .33  .110  
0.8  .37  .140  Large Effect  
0.9  .41  .168  
≥ 1.0  .45  .200 
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Borenstein, M., Hedges, L. V., Higgins, J. P. T., & Rothstein, H. R. (2009). Introduction to MetaAnalysis, Chapter 7: Converting Among Effect Sizes . Chichester, West Sussex, UK: Wiley.
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Morris, S. B. (2008). Estimating Effect Sizes From PretestPosttestControl Group Designs. Organizational Research Methods, 11(2), 364386. http://doi.org/10.1177/1094428106291059
Rosenthal, R. (1994). Parametric measures of effect size. In H. Cooper & L. V. Hedges (Eds.), The Handbook of Research Synthesis (231244). New York, NY: Sage.
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Thalheimer, W., & Cook, S. (2002, August). How to calculate effect sizes from published research articles: A simplified methodology. Retrieved March 9, 2014 from http://worklearning.com/effect_sizes.htm.
In case you need a reference to this page in a scientific paper, please used the following citation:
Lenhard, W. & Lenhard, A. (2016). Calculation of Effect Sizes. available: https://www.psychometrica.de/effect_size.html. Dettelbach (Germany): Psychometrica. DOI: 10.13140/RG.2.1.3478.4245