Choose between independent, paired, or one‑sample t‑tests; use Welch when variances or sample sizes differ; and always report effect sizes with confidence intervals.
Designs & When to Use
- Independent samples: two unrelated groups
- Paired: repeated measures or matched pairs
- One‑sample: compare mean to a reference value
Assumptions
- Approximately normal residuals (robust for n ≥ 30/group)
- Independent observations by design
- Equal variances for pooled t‑test; if not, use Welch
Effect Sizes
- Cohen’s d or Hedges’ g with 95% CIs
- Report alongside p‑values to communicate practical magnitude
Alternatives
- Mann‑Whitney U (independent) or Wilcoxon signed‑rank (paired) for strong non‑normality
- Bootstrap CIs when assumptions are doubtful