- Why?
- Correlation
*r* - Standardized Difference
*d*(Cohen’s*d*) - Odds Ratio
- Coefficient of determination (R
^{2}) - Omega / Eta / Epsilon Squared
- Interpertation of other Indices
- References

The metrics used in statistics (indices of fit, model performance or parameter estimates) can be very abstract. A long experience is required to intuitively * “feel”* the meaning of their values. In order to facilitate the understanding of the results they are facing, many scientists use (often implicitly) some set of

One of the most famous interpretation grids was proposed by **Cohen (1988)** for a series of widely used indices, such as the correlation **r** (*r* = .20, small; *r* = .40, moderate and *r* = .60, large) or the **standardized difference** (*Cohen’s d*). However, there is now a clear evidence that Cohen’s guidelines (which he himself later disavowed; Funder, 2019) are much too stringent and not particularly meaningful taken out of context (Funder and Ozer 2019). This led to the emergence of a literature discussing and creating new sets of rules of thumb.

Although **everybody agrees on the fact that effect size interpretation in a study should be justified with a rationale** (and depend on the context, the field, the literature, the hypothesis, etc.), these pre-baked rules can nevertheless be useful to give a rough idea or frame of reference to understand scientific results.

The package ** effectsize** implements such sets of rules of thumb for a variety of indices in a flexible and explicit fashion, helping you understanding and reporting your results in a scientific yet meaningful way. Again, readers should keep in mind that these thresholds, cited and used as they may be,

Moreover, some authors suggest the counter-intuitive idea that *very large effects*, especially in the context of psychological research, is likely to be a “gross overestimate that will rarely be found in a large sample or in a replication” (Funder and Ozer 2019). They suggest that smaller effect size are worth taking seriously (as they can be potentially consequential), as well as more believable.

**r < 0.05**- Tiny**0.05 <= r < 0.1**- Very small**0.1 <= r < 0.2**- Small**0.2 <= r < 0.3**- Medium**0.3 <= r < 0.4**- Large**r >= 0.4**- Very large

Gignac’s rules of thumb are actually one of few interpretation grid justified and based on actual data, in this case on the distribution of effect magnitudes in the litterature.

**r < 0.1**- Very small**0.1 <= r < 0.2**- Small**0.2 <= r < 0.3**- Moderate**r >= 0.3**- Large

**r < 0.1**- Very small**0.1 <= r < 0.3**- Small**0.3 <= r < 0.5**- Moderate**r >= 0.5**- Large

**r < 0.2**- Very weak**0.2 <= r < 0.4**- Weak**0.4 <= r < 0.6**- Moderate**0.6 <= r < 0.8**- Strong**r >= 0.8**- Very strong

The standardized difference can be obtained through the standardization of linear model’s parameters or data, in which they can be used as indices of effect size.

**d < 0.2**- Very small**0.2 <= d < 0.5**- Small**0.5 <= d < 0.8**- Medium**d >= 0.8**- Large

**d < 0.1**- Tiny**0.1 <= d < 0.2**- Very small**0.2 <= d < 0.5**- Small**0.5 <= d < 0.8**- Medium**0.8 <= d < 1.2**- Large**1.2 <= d < 2**- Very large**d >= 2**- Huge

Gignac’s rules of thumb are actually one of few interpretation grid justified and based on actual data, in this case on the distribution of effect magnitudes in the literature. These is in fact the same grid used for *r*, based on the conversion of *r* to *d*:

**d < 0.2**- Very small**0.2 <= d < 0.41**- Small**0.41 <= d < 0.63**- Moderate**d >= 0.63**- Large

Odds ratio, and *log* odds ratio, are often found in epidemiological studies. However, they are also the parameters of * logistic* regressions, where they can be used as indices of effect size. Note that the (log) odds ratio from logistic regression coefficients are

Note that these apply to Odds *ratios*, so Odds ratio of 10 is as extreme as a Odds ratio of 0.1 (1/10).

**OR < 1.68**- Very small**1.68 <= OR < 3.47**- Small**3.47 <= OR < 6.71**- Medium- **OR >= 6.71 ** - Large

**OR < 1.44**- Very small**1.44 <= OR < 2.48**- Small**2.48 <= OR < 4.27**- Medium- **OR >= 4.27 ** - Large

This converts (log) odds ratio to standardized difference *d* using the following formula (Cohen 1988; Sánchez-Meca, Marı́n-Martı́nez, and Chacón-Moscoso 2003):

[ d = log(OR) \times \frac{\sqrt{3}}{\pi} ]

**R2 < 0.02**- Very weak**0.02 <= R2 < 0.13**- Weak**0.13 <= R2 < 0.26**- Moderate**R2 >= 0.26**- Substantial

**R2 < 0.1**- Negligible**R2 >= 0.1**- Adequate

**R2 < 0.19**- Very weak**0.19 <= R2 < 0.33**- Weak**0.33 <= R2 < 0.67**- Moderate**R2 >= 0.67**- Substantial

**R2 < 0.25**- Very weak**0.25 <= R2 < 0.50**- Weak**0.50 <= R2 < 0.75**- Moderate**R2 >= 0.75**- Substantial

The Omega squared is a measure of effect size used in ANOVAs. It is an estimate of how much variance in the response variables are accounted for by the explanatory variables. Omega squared is widely viewed as a lesser biased alternative to eta-squared, especially when sample sizes are small.

**ES < 0.01**- Very small**0.01 <= ES < 0.06**- Small**0.16 <= ES < 0.14**- Medium- **ES >= 0.14 ** - Large

These are applicable to one-way ANOVAs, or to *partial* Eta / Omega / Epsilon Squared in a multi-way ANOVA.

**ES < 0.02**- Very small**0.02 <= ES < 0.13**- Small**0.13 <= ES < 0.26**- Medium**ES >= 0.26**- Large

`effectsize`

also offers functions for interpreting other statistical indices:

`interpret_gfi()`

,`interpret_agfi()`

,`interpret_nfi()`

,`interpret_nnfi()`

,`interpret_cfi()`

,`interpret_rmsea()`

,`interpret_srmr()`

,`interpret_rfi()`

,`interpret_ifi()`

, and`interpret_pnfi()`

for interpretation CFA / SEM goodness of fit.`interpret_p()`

for interpretation of*p*-values.`interpret_direction()`

for interpretation of direction.`interpret_bf()`

for interpretation of Bayes factors.`interpret_rope()`

for interpretation of Bayesian ROPE tests.`interpret_ess()`

and`interpret_rhat()`

for interpretation of Bayesian diagnostic indices.

Chen, Henian, Patricia Cohen, and Sophie Chen. 2010. “How Big Is a Big Odds Ratio? Interpreting the Magnitudes of Odds Ratios in Epidemiological Studies.” *Communications in Statistics—Simulation and Computation* 39 (4): 860–64.

Chin, Wynne W, and others. 1998. “The Partial Least Squares Approach to Structural Equation Modeling.” *Modern Methods for Business Research* 295 (2): 295–336.

Cohen, Jacob. 1988. “Statistical Power Analysis for the Social Sciences.”

———. 1992. “A Power Primer.” *Psychological Bulletin* 112 (1): 155.

Evans, James D. 1996. *Straightforward Statistics for the Behavioral Sciences.* Thomson Brooks/Cole Publishing Co.

Falk, R Frank, and Nancy B Miller. 1992. *A Primer for Soft Modeling.* University of Akron Press.

Field, Andy. 2013. *Discovering Statistics Using Ibm Spss Statistics*. sage.

Funder, David C, and Daniel J Ozer. 2019. “Evaluating Effect Size in Psychological Research: Sense and Nonsense.” *Advances in Methods and Practices in Psychological Science*, 2515245919847202.

Gignac, Gilles E, and Eva T Szodorai. 2016. “Effect Size Guidelines for Individual Differences Researchers.” *Personality and Individual Differences* 102: 74–78.

Hair, Joe F, Christian M Ringle, and Marko Sarstedt. 2011. “PLS-Sem: Indeed a Silver Bullet.” *Journal of Marketing Theory and Practice* 19 (2): 139–52.

Sawilowsky, Shlomo S. 2009. “New Effect Size Rules of Thumb.”

Sánchez-Meca, Julio, Fulgencio Marı́n-Martı́nez, and Salvador Chacón-Moscoso. 2003. “Effect-Size Indices for Dichotomized Outcomes in Meta-Analysis.” *Psychological Methods* 8 (4): 448.