Definition

Sample size

The number of participants or units in a study — determined by power analysis (quantitative) or saturation (qualitative), not convention.

Sample size is a calculated answer to "how much evidence do I need?": for detecting effects, power analysis solves n from the smallest effect of interest, alpha, and target power; for estimating proportions, the margin of error drives it; for interviews, saturation logic applies.

Too small wastes everyone’s time (real effects missed, estimates unstable); too large wastes resources and can make trivial effects "significant". Reviewers and ethics boards now expect the justification in the protocol, not the discussion.