Computing effect sizes
Cohen's d which is used for t-tests may be computed with a calculator here and here or using free PC downloadable software. This can also be calculated in EXCEL (see here) or using the pooled standard deviation given formula 1a in here. A suggestion for computing an overall Cohen's d comparing groups (over time) within each subject.
SPSS computes partial eta-squared, Partial eta2 , on request using ANOVAs. If using General Linear Model>univariate or General Linear Model>Repeated Measures click options and select Estimates of Effect Size. An extra column in the outputted anova tables is produced showing partial eta-squareds of terms in the anova table. Partial eta-squared represents the proportion of variance not attributable to any of the other observed factors which is explained by the factor of interest.
For anovas, an alternative value Eta-squared eta2 , can also be calculated. This value is defined as the sum of squares for a particular effect divided by the total of all the sums of squares of effects in the analysis of variance table. It is suggested, however, that partial eta-squared be used for repeated measures analysis of variance and eta-squared for between subjects anovas (which feature just one error term in the anova). Field(2005) advocates only using effect sizes when comparing a difference between two groups in repeated measures anova.
An EXCEL spreadsheet calculator computes the one sample chi-square effect size measure, $$\omega$$.
- The Pearson correlation is, itself, an effect size.
Field (2005)(pp. 222-223) suggests evaluating a correlation based upon output from a logistic regression. This is based upon the Wald statistic which can give misleading results.
Field also suggests a nonparametric effect size for comparing two groups (see here).
You might find A guide to magnitudes of effect sizes and Calculating, Interpreting and Reporting Estimates of "Effect Size" useful. The following spreadsheet by Jamie DeCoster (2012) converts a single effect size, such as Cohen's d, to several others including Odds Ratios using references mentioned in the spreadsheet. One could also convert a partial eta-squared to a Cohen's d by regarding the partial eta-squared as a squared correlation. It follows square rooting the partial eta-squared and entering it in Jamie's spreadsheet as a r will then allow you to read off the Cohen's d. Jamie has written other EXCEL spreadsheet calculators here. Howell (2013) p.627-8 gives formulae for conversions of effect sizes such as odds ratios and correlations to Cohen's d.
References
Baguley T (2012) Serious Stats. A guide to advanced statistics for the behavioral sciences. Palgrave MacMillan:New York. R code and formulae for a range of commonly used effect sizes are in Chapter 7 on pages 235-276.
DeCoster J (2012) Spreadsheet for converting effect size measures. Available from: http://www.stat-help.com/spreadsheets/Converting%20effect%20sizes%202012-06-19.xls (accessed 04.09.2014)
Field A (2005) Discovering statistics using SPSS Sage:London.
Howell DC (2013) Statistical methods for psychologists. 8th Edition. International Edition. Wadsworth:Belmont,CA.
Vacha-Haase T & Thompson B (2004) How to estimate and interpret various effect sizes. Journal of Counseling Psychology 51(4) 473-481. Details computing effect sizes in SPSS for methods including ANOVA and regression as well as showing conversion formulae expressing one effect size in terms of another.