Within-subjects and repeated-measures designs
Observe change or contrasts within the same person
Within-subjects designs expose each participant to multiple conditions; repeated-measures designs observe the same unit across conditions or times. These overlap, but repeated observational measurements need not involve an experiment. Participants can serve as their own comparators, while period, learning and carryover effects need planning. Analysis must retain within-unit dependence.
Use for reversible manipulations or repeated trajectories when participant-level variation is substantial. Check that earlier exposure will not irreversibly change later responses and that the proposed observation schedule is feasible.
Strengths
- Controls stable participant differences in comparisons
- Can estimate within-person change efficiently
Limitations
- Carryover and practice can confound condition effects
- Missing visits and unequal timing complicate analysis
Know the boundary
A large number of trials from one participant is not a large independent participant sample.