Calculate effect size r from t or F statistics
r is reported as an absolute value (direction is not preserved). Interpretation uses Cohen's (1988) benchmarks: 0.1 small, 0.3 medium, 0.5 large. The F-statistic formula is valid only for a single-df effect (df between = 1, e.g. a two-group comparison); do not use it for an omnibus ANOVA with 3+ groups.
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