Sample Size
What Happens to Power with More Groups? Contrast-Focused Sample Planning
Link sample-size planning for ANOVA, planned contrasts, unequal allocation and multiplicity to the actual primary hypothesis.
Admin
June 20, 2026
8 min read
The number of groups is not enough
Power differs for an omnibus test, a planned contrast and pairwise comparisons. Prespecify the primary hypothesis.
Practical framework
Define omnibus versus contrast target.
Account for allocation and cost.
Include multiplicity for multiple primary tests.
Inflate transparently for attrition and design effects.
Reporting
Report per-group and total samples, allocation and powered comparison.
Common mistakes
Blindly dividing total N.
Ignoring planned post-hoc tests.
Ignoring unequal-allocation efficiency.
References
Accessed 20 June 2026. General methodological information.