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| Taken from Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale, NJ: Lawrence Erlbaum Associates. $$\eta^text{2}$$ is also mentioned by Tabachnick and Fidell (2007). | Taken from Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale, NJ: Lawrence Erlbaum Associates. |
Rules of thumb on magnitudes of effect sizes
Taken from Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale, NJ: Lawrence Erlbaum Associates.
Effect Size |
Use |
Small |
Medium |
Large |
|||
Correlation |
|
0.1 |
0.3 |
0.5 |
|||
$$\eta^text{2}$$ |
regression including anovas |
0.01 |
0.06 |
0.14 |
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Cohen's f |
regression including anovas |
0.1 |
0.25 |
0.40 |
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Cohen's d |
t-tests |
0.2 |
0.5 |
0.8 |
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Cohen's $$\omega$$ |
chi-square |
0.1 |
0.3 |
0.5 |
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Odds Ratios |
2 by 2 tables |
1.5 |
3.5 |
9.0 |
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Also:Haddock et al (1998) state that $$\sqrt{3/\pi}$$ multiplied by the log of the odds ratio is a standardised difference equivalent to Cohen's d.
[http://www.sportsci.org/resource/stats/effectmag.html Further details on the derivation of the Odds Ratio effect sizes]
- [:FAQ/baserat: Cohen's d adjusted for base rates]
- [:FAQ/dunlap: A quick guide to choice of sample sizes for Cohen's effect sizes]
Definitions
$$\mbox{Cohen's d} = \frac{\mbox{difference between a pair of group means}}{\mbox{averaged group standard deviation}}$$
$$\eta^text{2} = \frac{\mbox{ SS(effect)} }{\mbox{Sum of SS(effects) + SS(error for those effects)} $$
$$\mbox{Cohen's f} = \sqrt{\frac{\etatext{2} }{1 - \etatext{2}} }$$
There is also a $$\mbox{Partial } \eta^text{2} = \frac{\mbox{ SS(effect)} }{\mbox{SS(effect) + SS(error for that effect)} $$
$$\omega = \sqrt{ \sum_text{1}text{no. groups} \frac{\mbox{(observed proportion - expected proportion)}text{2}}{ \mbox{ expected proportion}}}$$
* [wiki:FAQ/oddsr Odds Ratio definition]
References
Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale, NJ: Erlbaum
Haddock, CK, Rinkdskopf, D. & Shadish, C. (1998) Using odds ratios as effect sizes for meta-analysis of dichotomous data: A primer on methods and issues. Psychological Methods 3 339-353.
Tabachnick, BG and Fidell, LS (2007) Using multivariate statistics. Fifth Edition. Pearson Education:London.