Computes Cohen's d effect size for a two-group comparison as abs(mean1 - mean2) / sd_pooled, with sd_pooled = sqrt((sd1^2 + sd2^2) / 2). This is the same formula used inline by report_ttests, extracted here as a standalone, reusable function so it can be called on its own.
Arguments
- formula
a formula of the form dv ~ group, same as used by stats::t.test's formula method. The grouping variable must have exactly two levels. Note: as with stats::t.test's formula method, paired tests are not supported here; use stats::t.test(x, y, paired = TRUE) directly for that case.
- data
data frame containing the variables in formula, same as stats::t.test's data argument.
Note
effect size
Very small 0.01 Sawilowsky (2009)
Small 0.20 Cohen (1988)
Medium 0.50 Cohen (1988)
Large 0.80 Cohen (1988)
Very large 1.20 Sawilowsky (2009)
Huge 12.0 Sawilowsky (2009)
Examples
compute_cohens_d(
formula = bp_before ~ agegrp,
data = df_blood_pressure[df_blood_pressure$agegrp %in% c("30-45", "46-59"), ]
)
#> [1] 0.3287838
effectsize::cohens_d(
bp_before ~ agegrp,
data = df_blood_pressure[df_blood_pressure$agegrp %in% c("30-45", "46-59"), ],
pooled_sd = TRUE
)
#> Cohen's d | 95% CI
#> -------------------------
#> -0.33 | [-0.77, 0.11]
#>
#> - Estimated using pooled SD.