Hedges' g standardized mean difference — hedges

Computes the standardized mean difference (Hedges' g) between a treatment and a comparison group for a single numeric covariate, using the pooled within-group standard deviation and the small-sample correction factor used by the What Works Clearinghouse (WWC).

Usage

hedges_g(x, treatment, na.rm = TRUE)

Arguments

x: Numeric vector of covariate values.
treatment: Vector the same length as x identifying group membership. Must have exactly two unique non-missing values. The larger value (e.g. 1, TRUE, or the second sorted level) is treated as the treatment group; the other as the comparison group.
na.rm: Logical; drop rows where x or treatment is NA. Default TRUE.

Value

A single numeric value: Hedges' g. Positive when the treatment group mean exceeds the comparison group mean.

Details

The correction factor is \(\omega = 1 - 3 / (4N - 9)\), where \(N = n_{treatment} + n_{comparison}\).

References

What Works Clearinghouse (2022). Procedures Handbook (Version 5.0). U.S. Department of Education.

Examples

x <- c(5, 6, 7, 4, 5, 6)
g <- c(1, 1, 1, 0, 0, 0)
hedges_g(x, g) # 0.8
#> [1] 0.8