For the reader to fully understand the importance of your findings, it is almost always necessary to include some index of effect size or strength of relationship in your Results section (APA 2001).
p is dependent on sample size
N mean SD t p Choen's d
Experiment 1 1.52 > 0.05 0.56
Group A 15 25 9
Group B 15 20 9
Experiment 2 2.15 < 0.05 0.56
Group A 30 25 9
Group B 30 20 9
pvalue can not show: uncertaintity, direction and intensity of effect
Effect size
ttest: d
ANOVA: f, f^{2}, η, η^{2}
η^{2} = f^{2}/(1 + f^{2})
ANCOVA: f
(multiple) regression: f
MANOVA: f^{2}
χ^{2}test: w
Confidence interval of effect size
Interval estimates: should be given for any effect sizes involving principal outcomes
R packages
rpsychi
compute.es
MBESS

Table 1. Guidelines for calculating, reporting, and interpreting effect sizes (ESs) (Durlak 2009)
 Choose the most suitable type of effect based on the purpose, design, and outcome(s) of a research study
 Provide the basic essential data for the major variables^{a}
 for group designs, present means, standard deviations, and sample size for all groups on all outcomes at all time points of measurement
 for correlational studies, provide a complete correlation matrix at all time points of measurement
 for dichotomous outcomes, present the cell frequencies or proportions and the sample sizes for all groups
 Be explicit about the type of ES that is used
 Present the effects for all outcomes regardless of whether or not statistically significant findings have been obtained
 Specify exactly how effects were calculated by giving a specific reference or providing the algebraic equation used
 Interpret effects in the context of other research
 the best comparisons occur when the designs, types of outcomes, and methods of calculating effects are the same across studies
 evaluate the magnitude of effect based on the research context and its practical or clinical value
 if effects from previous studies are not presented, strive to calculate some using the procedures described here and in the additional references
 use Cohen's (1988) benchmarks, only if comparisons to other relevant research are impossible
^{a} These data have consistently been recommended as essential information in any report, but they also can serve a useful purpose in subsequent research if readers need to make any adjustments to your calculations based on new analytic strategies or want to conduct more sophisticated analyses. For example, the data from a complete correlation matrix is needed for conducting metaanalytic mediational analyses.
Table 1. Strategies for obtaining effect sizes for selected SPSS analyses (VachaHaase & Thompson 2004)
 Contingency table (r or odds ratio): Run the CROSSTABS procedure and select the desired effect from the STATISTICS submenu
 Independent t test (d, η^{2}, or ω^{2}): Compute a Cohen's d by hand. Or, run the analysis as a oneway ANOVA using the GLM program; click on the OPTION requesting an effect size to obtain η^{2}. Use the Hay's correction formula (ω^{2}) if an adjusted estimate is desired
 ANOVA (η^{2} or ω^{2}): Run the analysis as an ANOVA using the GLM program; click on the OPTION requesting an effect size to obtain η^{2}. Use the Hay's correction formula by hand if an adjusted estimate is desired
 Regression (R^{2} or R^{2}*) Run the REGRESSION procedure. Both the uncorrected R^{2} and the corrected variance accounted for (R^{2}*) estimates are displayed, by default
 MANOVA (multivariate η^{2} or ω^{2}): Run the analysis as a MANOVA using the GLM program; click on the OPTION requesting an effect size to obtain η^{2}. A corrected estimate, multivariate ω^{2}, (Tatsuoka 1973), can be computed by hand
 Descriptive discriminant analysis (multivariate η^{2} or ω^{2}): Run the analysis as a MANOVA using the GLM program; click on the OPTION requesting an effect size to obtain η^{2}. A corrected estimate, multivariate ω^{2} (Tatsuoka 1973), can be computed by hand.
 Canonical correlation analysis (R_{c}^{2} or R_{c}^{2}*): Run the analysis in the MANOVA procedure using the syntax suggested by Thompson (2000). The R_{c}^{2} is reported. Apply the Ezekiel correction by hand if a corrected value (R_{c}^{2}*) is desired
Note. ANOVA = analysis of variance; GLM = general linear model; MANOVA = multivariate analysis of variance
