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Cohen's d = t (n1 + n2) / Sqrt(df)Sqrt{n1}Sqrt(n2) | Cohen's d = t (n1 + n2) / [Sqrt(df) Sqrt{n1) Sqrt(n2)] |
How do I convert a t-statistic into an effect size?
One sample t
Since Sqrt{n(mean)) / sd = t
Cohen's d = t/Sqrt(n) = (mean-constant)/sd
This value of Cohen's d is used by Lenth (2006) in his one sample and (paired sample) t-test option.
Two sample unpaired t
Rosenthal (1994) states for sample sizes using an unpaired t-test n1, n2},
Cohen's d = t (n1 + n2) / [Sqrt(df) Sqrt{n1) Sqrt(n2)]
When n1 = n2
Cohen's d = 2t / df (See also Howell (2013), p.649)
Paired t
Baguley (2012, p.271) gives a formula, amongst other conversion formulae, for converting a paired t to d using a joint group size equal to n:
d = (difference in means) / (sd of (population) difference in means) where the population sd is 1/(n-1) (sum of squared deviations from the average difference).
d, above, can also be expressed as
d = t Sqrt(1/n) Sqrt(n/(n-1)) = difference in means / sd of sample difference [n/(n-1)] (see p.248 of Baguley (2012)) where the sd of the sample difference is the square root of 1/n (sum of the squared deviations of each difference from the overall sample mean difference) as defined on page 23 of Baguley (2012) and Sqrt(n}/(n-1) is the correction factor for estimating a population sd from a sample sd (pages 26-27 of Baguley).
2 way interaction
Abelson and Prentice (1997) suggest a way of converting a F statistic from a two-way interaction into Cohen's d:
Cohen's d = Sqrt(2) Sqrt(F)/Sqrt(n)
where n is the assumed equal number of observations for each combination of the two factors. If these are unequal then we use the harmonic mean of the sample sizes.
The two sample t-test with equal sample sizes is a special case since t equals $$\sqrt{F}$$ and df is made equal to 2n.
Pearson Correlation
Rosenthal (1994) also gives a conversion formula to turn a t-statistic into a correlation
Correlation = square root of $$[\mbox{t}2$$
divided by
$$(\mbox{t}2 + df)]$$
General Conversions
Jamie DeCoster has written a spreadsheet to convert a range of commonly used effect sizes such as Cohen's d, Pearson's r and odds ratios.
References
Abelson, R. P. and Prentice, D. A. (1997) Contrast tests of interaction hypotheses. Psychological Methods 2(4) 315-328.
Baguley, T. (2012) Serious Stats. A guide to advanced statistics for the behavioral sciences. Palgrave Macmillan:New York. In addition to those mentioned above, Chapter 7 gives some conversion formulae including converting from r to g, where g is an effect size estimator which is very closely related to d.
Howell, D. C. (2013) Statistical methods for psychologists. 8th Edition. International Edition. Wadsworth:Belmont, CA.
Lenth, R. V. (2006) Java Applets for Power and Sample Size [Computer software]. Retrieved month day, year, from http://www.stat.uiowa.edu/~rlenth/Power.
Rosenthal, R. (1994) Parametric measures of effect size. In H. Cooper and L.V. Hedges (Eds.). The handbook of research synthesis. New York: Russell Sage Foundation.