Description

These functions are convenience functions to convert F and t test statistics to partial Eta squared, (η{_p}^2), Omega squared (ω{_p}^2), Epsilon squared (ε{_p}^2; an alias for the adjusted Eta squared) and Cohen's f. These are useful in cases where the various Sum of Squares and Mean Squares are not easily available or their computation is not straightforward (e.g., in liner mixed models, contrasts, etc.). For test statistics derived from lm and aov models, these functions give exact results. For all other cases, they return close approximations.
See Effect Size from Test Statistics vignette.

Usage

Arguments

Details

These functions use the following formulae:

η_p^2 = \frac{F \times df_{num}}{F \times df_{num} + df_{den}}

ε_p^2 = \frac{(F - 1) \times df_{num}}{F \times df_{num} + df_{den}}

ω_p^2 = \frac{(F - 1) \times df_{num}}{F \times df_{num} + df_{den} + 1}

f_p = √{\frac{η_p^2}{1-η_p^2}}

For t, the conversion is based on the equality of t^2 = F when df_{num}=1.

Choosing an Un-Biased Estimate

Both Omega and Epsilon are unbiased estimators of the population Eta. But which to choose? Though Omega is the more popular choice, it should be noted that:

1. The formula given above for Omega is only an approximation for complex designs.

2. Epsilon has been found to be less biased (Carroll & Nordholm, 1975).

Value

A data frame with the effect size(s) between 0-1 (Eta2_partial, Epsilon2_partial, Omega2_partial, Cohens_f_partial or Cohens_f2_partial), and their CIs (CI_low and CI_high). (Note that for ω_p^2 and ε_p^2 it is possible to compute a negative number; even though this doesn't make any practical sense, it is recommended to report the negative number and not a 0).

Confidence Intervals

Unless stated otherwise, confidence intervals are estimated using the Noncentrality parameter method; These methods searches for a the best non-central parameters (ncps) of the noncentral t-, F- or Chi-squared distribution for the desired tail-probabilities, and then convert these ncps to the corresponding effect sizes. (See full effectsize-CIs for more.)

CI Contains Zero

For positive only effect sizes (Eta squared, Cramer's V, etc.; Effect sizes associated with Chi-squared and F distributions), special care should be taken when interpreting CIs with a lower bound equal to 0, and even more care should be taken when the upper bound is equal to 0 (Steiger, 2004; Morey et al., 2016). For example:

Note

Adj. η_p^2 is an alias for ε_p^2.

References

• Albers, C., & Lakens, D. (2018). When power analyses based on pilot data are biased: Inaccurate effect size estimators and follow-up bias. Journal of experimental social psychology, 74, 187-195. doi: 10.31234/osf.io/b7z4q

• Carroll, R. M., & Nordholm, L. A. (1975). Sampling Characteristics of Kelley's epsilon and Hays' omega. Educational and Psychological Measurement, 35(3), 541-554.

• Cumming, G., & Finch, S. (2001). A primer on the understanding, use, and calculation of confidence intervals that are based on central and noncentral distributions. Educational and Psychological Measurement, 61(4), 532-574.

• Friedman, H. (1982). Simplified determinations of statistical power, magnitude of effect and research sample sizes. Educational and Psychological Measurement, 42(2), 521-526. doi: 10.1177/001316448204200214

• Mordkoff, J. T. (2019). A Simple Method for Removing Bias From a Popular Measure of Standardized Effect Size: Adjusted Partial Eta Squared. Advances in Methods and Practices in Psychological Science, 2(3), 228-232. doi: 10.1177/2515245919855053

• Morey, R. D., Hoekstra, R., Rouder, J. N., Lee, M. D., & Wagenmakers, E. J. (2016). The fallacy of placing confidence in confidence intervals. Psychonomic bulletin & review, 23(1), 103-123.

• Steiger, J. H. (2004). Beyond the F test: Effect size confidence intervals and tests of close fit in the analysis of variance and contrast analysis. Psychological Methods, 9, 164-182.

See Also

eta_squared() for more details.

Other effect size from test statistic: chisq_to_phi(), t_to_d()

Examples