Fifteen-second views divided by three-second views - of the people your hook stopped, how many the body of the ad kept. Hook rate and hold rate together split creative diagnosis in two: the hook earns attention, the hold proves the narrative deserves it. A strong hook with a weak hold means the opening is writing cheques the story cannot cash. This is what makes modular creative work: when hold rate is healthy but the hook has fatigued, keep the proven body and cut new openings onto it - three fresh variants the same afternoon, off footage you already own.
Benchmark. Target 15-20%+; below 10% the narrative is losing them after the hook did its job. Measure it per body and per angle, not just per ad, so you learn which modules are pulling the weight.