# Cohort Analysis

## Objective

Group users by when they started and watch how each group behaves over time.

A retention grid that buckets users by a qualifying moment and tracks what each bucket does in the periods that follow.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Define the qualifying event that puts a user in a cohort, and freeze their membership at that moment. A cohort that re-sorts itself as users change is not a cohort.
2. Support both calendar buckets (users who signed up in March) and rolling buckets (day 0, day 7, day 30 relative to each user's own start).
3. Render the result as a triangular grid: cohorts down, periods across, with each cell showing both the count and the rate.
4. Read from the app's existing event stream. Event capture is owned by Event Tracking — extend that rather than instrumenting a second pipeline.
5. Do NOT restate ordered step-conversion logic here; that belongs to Funnel Analytics. This feature is about time, not sequence.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- A user who qualifies twice must be handled by a stated rule — first qualification only, or a separate row per qualification. Silently double-counting inflates every cell.
- The most recent cohort has not lived through a full period. Mark partial periods rather than plotting them as a collapse in retention.
- Late-arriving events change historical cells. Decide whether cells are recomputed or frozen, and say which on the page.
- Calendar buckets need a stated time zone. UTC and the viewer's local zone put the same user in different months.
- A cohort with a handful of users produces meaningless percentages. Suppress or flag rates below a minimum cohort size.
- Deleted or merged accounts must not silently shrink historical cohorts — retention would appear to improve.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Cohort membership is fixed at the qualifying moment and does not change retroactively.
- [ ] Both calendar and rolling bucketing are available and clearly labelled.
- [ ] Users who qualify more than once are handled by a documented rule.
- [ ] Partial and incomplete periods are visibly marked, not plotted as data.
- [ ] Small cohorts are flagged or suppressed rather than shown as noisy percentages.
- [ ] The bucketing time zone is stated on the report.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.