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Bayes effect size delta

edited September 2017 in JASP & BayesFactor

Hi there,

Two questions about the effect size used in JASP:

  1. What is an effect size (delta)? Is it Glass's delta?
  2. How do you convert Cohen's d into delta? (this is straightforward if delta is Glass's delta)



PS. Thank you Team JASP - the program is fantastic!!


  • Hi Hannah,
    I assume you are talking about the t-test. Delta is the population version of the sample Cohen's d. So it is just the mean population difference divided by the population sd. You can find a detailed description in (and also in the PBR t-test paper by Rouder et al., which is cited in the Ly paper).

  • Very helpful - thank you! And thanks for your speedy reply! Much appreciated:)

  • When using the BEST-package in R the effect size I get, has a mode of -1.55 with a 95% CI of -2.36, -0.528. While in JASP the median is -0.685 with a 95% CI -1.211, -0.184. This on the exact same dataframe. As far as I know, the effect size in BEST is δ as well. Can anybody explain te difference?

  • This is just a wild guess, but I think that BEST uses a t-likelihood, not a normal likelihood. This *may* explain the difference

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