84,395 Users, Effect Size 0.7: Why a Pretty Number Doesn't Mean a Self-Hypnosis App 'Works'
An npj Digital Medicine study analyzed 84,395 users and 282,893 sessions on the self-hypnosis app Reveri. Self-rated stress dropped with a Cohen's d of −0.71 to −0.78 (a 'large' effect). But it's a retrospective observational study with no control group and a single-item Likert measure — a great case study in why 'big N + big effect' still can't prove causation.