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| Field (2005) suggests an effect size associated with performing nonparametric group tests. This is analogous to Cohen's d for parametric group testing. In particular on pages 531-2 of his book Field suggests computing a correlation: | Field (2005) suggests an effect size associated with performing nonparametric group tests. This is analogous to Cohen's d for parametric group testing. In particular on pages 531-2 of his book Field suggests using the formula of Rosenthal (1991, p.19) to compute a correlation: |
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| Field A (2005) Discovering Statistics Using SPSS. Second Edition. Sage:London | Field A (2005) Discovering statistics using SPSS. Second Edition. Sage:London Rosenthal R (1991) Meta-analytic procedures for social research (revised). Sage:Newbury Park,CA. |
A two group nonparametric effect size
Field (2005) suggests an effect size associated with performing nonparametric group tests. This is analogous to Cohen's d for parametric group testing. In particular on pages 531-2 of his book Field suggests using the formula of Rosenthal (1991, p.19) to compute a correlation:
correlation = z /sqrt(N)
where z is the z statistic that SPSS produces when you do the Mann-Whitney or Wilcoxon tests under analyze>nonparametrics and N is the size of the study.
This is a correlation so can be compared to the rules of thumb suggested by Cohen with suggested thresholds of 0.1, 0.3 and 0.5 for small, medium and large magnitudes respectively.
References
Field A (2005) Discovering statistics using SPSS. Second Edition. Sage:London
Rosenthal R (1991) Meta-analytic procedures for social research (revised). Sage:Newbury Park,CA.