Diagnose a Metric Drop

A large consumer web/mobile product sees its key business metric drop materially and suddenly. Assume: the sitewide purchase conversion rate fell by 12 % relative (e.g., 10.0 % → 8.8 %) starting Tuesday 10:00 AM and persisting for several days. Your task is to: 1) List plausible causes for the performance drop. 2) For each cause, describe specific analyses you would run to validate or falsify it (be concrete about cuts, comparisons, and supporting logs/metadata). 3) Choose your main hypothesis and design an A/B test to confirm it, including hypothesis and treatment/control design, randomization unit and exposure, sample size, duration, and analysis plan. 4) Specify primary and guardrail metrics you would track and why.