effsize (version 0.6.4)

# cohen.d: Cohen's d and Hedges g effect size

## Description

Computes the Cohen's d and Hedges'g effect size statistics.

## Usage

cohen.d(d, ...)
"cohen.d"(formula,data=list(),...)
"cohen.d"(d,f,pooled=TRUE,paired=FALSE, na.rm=FALSE, hedges.correction=FALSE, conf.level=0.95,noncentral=FALSE, ...)

## Arguments

d
a numeric vector giving either the data values (if f is a factor) or the treatment group values (if f is a numeric vector)
f
either a factor with two levels or a numeric vector of values
pooled
a logical indicating whether compute pooled standard deviation or the whole sample standard deviation
paired
a logical indicating whether to consider the values as paired
na.rm
logical indicating whether NAs should be removed before computation; if paired==TRUE then all incomplete pairs are removed.
hedges.correction
logical indicating whether apply the Hedges correction
conf.level
confidence level of the confidence interval
formula
a formula of the form y ~ f, where y is a numeric variable giving the data values and f a factor with two levels giving the corresponding groups
data
an optional matrix or data frame containing the variables in the formula formula. By default the variables are taken from environment(formula).
noncentral
logical indicating whether to use non-central t distributions for computing the confidence interval.
...
further arguments to be passed to or from methods.

## Value

A list of class effsize containing the following components: containing the following components:

## Details

When f in the default version is a factor or a character, it must have two values and it identifies the two groups to be compared. Otherwise (e.g. f is numeric), it is considered as a sample to be compare to d.

In the formula version, if f is expected to be a factor, if that is not the case it is coherced to a factor and a warning is issued.

The function computes the value of Cohen's d statistics (Cohen 1988). If required (hedges.correction==TRUE) the Hedges g statistics is computed instead (Hedges and Holkin, 1985).

When paired is set, the effect size is computed using the approach suggested in (Gibbons et al. 1993).

The computation of the CI requires the use of non-central Student-t distributions that are used when noncentral==TRUE; otherwise a central distribution is used.

Also a quantification of the effect size magnitude is performed using the thresholds define in Cohen (1992). The magnitude is assessed using the thresholds provided in (Cohen 1992), i.e. |d|<0.2 "negligible", |d|<0.5 "small", |d|<0.8 "medium", otherwise "large"

The variance of the d is computed using the conversion formula reported at page 238 of Cooper et al. (2009):

$$S^2_d = \left( \frac{n_1+n_2}{n_1 n_2} + \frac{d^2}{2 df}\right) \left( \frac{n_1+n_2}{df} \right)$$

## References

Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). New York:Academic Press.

Hedges, L. V. & Olkin, I. (1985). Statistical methods for meta-analysis. Orlando, FL: Academic Press.

Cohen, J. (1992). A power primer. Psychological Bulletin, 112, 155-159.

The Handbook of Research Synthesis and Meta-Analysis (Cooper, Hedges, & Valentine, 2009)

David C. Howell (2010). Confidence Intervals on Effect Size. Available at: https://www.uvm.edu/%7Edhowell/methods7/Supplements/Confidence%20Intervals%20on%20Effect%20Size.pdf

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, 633-649.

Gibbons, R. D., Hedeker, D. R., & Davis, J. M. (1993). Estimation of effect size from a series of experiments involving paired comparisons. Journal of Educational Statistics, 18, 271-279.

cliff.delta, VD.A, print.effsize

## Examples

Run this code
treatment = rnorm(100,mean=10)
control = rnorm(100,mean=12)
d = (c(treatment,control))
f = rep(c("Treatment","Control"),each=100)
## compute Cohen's d
## treatment and control
cohen.d(treatment,control)
## data and factor
cohen.d(d,f)
## formula interface
cohen.d(d ~ f)
## compute Hedges' g
cohen.d(d,f,hedges.correction=TRUE)


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