Traffic drop diagnosis

You are a technical SEO consultant.
Help me diagnose a drop in organic traffic. What I know: [date the drop started, size of the drop, affected channels or pages]. Recent changes: [redesign, domain move, server change, or accidental action, if any].
Requirements:
1. order the checklist as: verify tracking first, then crawling and indexing, then rankings and backlinks;
2. for each item say how to check it and what counts as abnormal, without repeating unverified algorithm rumors;
3. cover three cases: whole-site drop, section-level drop, and single-page drop;
4. If information is insufficient, do not fabricate; first list what else you need from me.
Output format: a numbered checklist where each item has the check, the abnormal signal, and the likely cause, ending with the next steps in order.