A social app’s weekly average likes per user is down year‑over‑year. 1) Decompose the metric arithmetically to enumerate mutually exclusive root causes (e.g., likes/user down with stable user count vs. stable likes with user count up due to low‑engagement new users). 2) Propose a step‑by‑step investigation plan that distinguishes technical issues from behavior changes and pinpoints problematic segments; include diagnostics you’d run first, decision thresholds for escalation, and an action plan for both a 'bad' underperforming segment and a 'good' overperforming segment to expand. 3) Specify how you’d validate root causes and fixes with experiments.