cohort-analysis

name	cohort-analysis
description	Builds and interprets a cohort retention table — defines the cohort and return event, reads where and when users drop off, compares cohorts over time, and converts the retention curve into a hypothesis and an action. Use when you say "build a cohort table," "why is retention dropping," "are newer signups sticking better," "where do users churn," or "what happened to the March cohort."
tier	quick
time	15-30 min
inputs	a retention/usage table by signup cohort
outputs	metrics/cohort-<segment>-<date>.md

Cohort Analysis

Group users by when they first joined, then track what fraction return in each period after. A cohort retention table turns a flat churn number into a curve you can read: where the cliff is, whether the curve flattens (a sticky core), and whether newer cohorts retain better than older ones.

Grounded in: Lean Analytics — Croll & Yoskovitz: cohort retention curves read against a benchmark, not zero. Go deeper (The Product Channel): Product Metrics

When to use this

• Retention or churn looks bad but you can't tell if it's getting better or worse over time.
• You shipped an onboarding, activation, or pricing change and want to see if it moved retention for users who joined after.
• You need to know where in the lifecycle users drop — day 1, week 1, month 2 — not just that they do.
• Leadership asks "are the users we're acquiring now any good?" and you need cohort-over-cohort evidence.
• You want to find your retention floor (the curve flattening) to size a real, durable user base vs. a leaky bucket.

Before you start (gather these)

• The cohort definition: what groups users (usually signup/first-purchase date) and the grain (daily, weekly, monthly).
• The return event: what "retained" means — opened app, completed a core action, paid again. Vanity logins are weak; a value-moment event is strong.
• The period grid: how you'll bucket time after joining (Day 0/1/7/30, or Week 0–8, or Month 0–6).
• The raw data: either a pre-built cohort table, or per-user rows of (join_date, activity_dates) you can pivot.

If the return event is undefined or the grain is unclear (2+ of these vague or missing), ASK 2–4 sharp questions first: What single action means a user got value? What's the natural usage frequency — daily, weekly, monthly? When does a user "start" (signup vs. first key action)? If data is already provided, proceed and state assumptions inline (e.g. "Assuming retained = ≥1 core action that week; cohort = signup week").

Process

1. Lock the definition before touching numbers. One return event, one start moment, one period grain matched to natural usage frequency (don't measure a monthly product daily). Write it as one sentence: "Cohort = [start], retained = [event] within [period]."
2. Build the table. Rows = cohorts (oldest at top), columns = periods since join (0, 1, 2…). Cell = % of that cohort's Period-0 size active in that period. Period 0 is always 100% (or define it as activated users). Keep an N column — never read a cohort with tiny N.
3. Find the cliff (scan EVERY adjacent-period drop). Don't presume it's P0→P1. Compute the drop between each adjacent pair (P0→P1, P1→P2, P2→P3…) and find the largest fall wherever it lands. Many products have a week-2 (or later) cliff, not a day-1 one. A P0→P1 cliff points at onboarding/activation; a later cliff points at a habit or value-depth problem. Whatever the biggest fall is, that's the cliff — name it by the period pair where it actually occurs.
4. Read across each row (the curve shape). Does it keep falling to zero (leaky bucket, no product-market fit) or flatten into a plateau (a retained core — that plateau % is your retention floor)?
5. Read down each column (cohort-over-cohort). Hold the period constant and compare cohorts. If newer cohorts show higher Period-1 or Period-3 retention, your recent changes are working. This is the diagonal-free way to see trend.
6. Find the inflection. Name the single period where the most users are lost relative to value delivered. That's where one intervention has the most leverage.
7. Segment if the curve is muddy. Split by acquisition channel, plan, or first action. Blended curves often hide one great segment and one terrible one.
8. Form a hypothesis, then an action. Convert the read into one testable sentence: "Users drop at [period] because [hypothesis]; if we [action], Period-[n] retention rises from [x]% to [target]%."

Output template

# Cohort Retention Analysis — [Product / Segment]

**Date:** [date] · **Analyst:** [name]
**Definition:** Cohort = users by [signup week]. Retained = [≥1 core action] in the period. Grain = [weekly].
**P0 denominator:** P0 = [all signups | activated only] — this choice changes every downstream %.
**Signup→P0 activation rate:** [z]% ([signups] → [activated]). *(Required when P0 = activated only — an activated-P0 table conceals tire-kicker churn that happens before activation. Report this rate or warn it's hidden.)*
**Window:** [cohorts from X to Y] · **Assumptions:** [stated inline]

## Retention table (% of cohort active)
| Cohort (size N) | P0 | P1 | P2 | P3 | P4 | P5 |
|---|---|---|---|---|---|---|
| [Wk of date] (N=[ ]) | 100% | [ ]% | [ ]% | [ ]% | [ ]% | [ ]% |
| [Wk of date] (N=[ ]) | 100% | [ ]% | [ ]% | [ ]% | [ ]% | — |
| [Wk of date] (N=[ ]) | 100% | [ ]% | [ ]% | — | — | — |
| **Avg (mature cohorts only)** | 100% | [ ]% | [ ]% | [ ]% | [ ]% | [ ]% |

*Avg row excludes partial/young cohorts (those without complete periods) — blending them in distorts the curve.*

**Benchmark band:** ~[X]% P1 / ~[Y]% floor — typical for [category, e.g. B2B SaaS vs. consumer social]. Read every number against this band, not against zero.

## What the curve says
- **The cliff (P[n]→P[n+1]):** [x]% drop — the largest adjacent fall, found by scanning every period pair (not assumed at P0→P1). [Onboarding/activation read if early; habit/value-depth read if later.]
- **Curve shape:** [Falls to zero ▸ leaky bucket] OR [flattens at ~[y]% by P[n] ▸ that's the retention floor].
- **Retention floor:** ~[y]% — the durable core. [Is that enough to build on?]
- **Cohort-over-cohort (read down a column):** P[n] retention went [x]% → [y]% across cohorts ▸ [improving / flat / decaying]. [Tie to a known ship.]
- **Inflection point:** Most users lost at **P[n]** relative to value delivered.

## Segment cut (if run)
| Segment | P1 | P3 | Floor | Read |
|---|---|---|---|---|
| [Channel / plan A] | [ ]% | [ ]% | [ ]% | [strong/weak] |
| [Channel / plan B] | [ ]% | [ ]% | [ ]% | [strong/weak] |


  Users drop at  because [specific cause].
  [the cells / comparison that support it].
  [one intervention targeting that period].
  P[n] retention rises from  for cohorts after [ship date].
  Re-pull this table in [N periods]; watch the [P n] column down the cohorts.

Avoid (anti-patterns)

• Reading a single blended retention number instead of the curve — it hides whether you're improving or decaying over cohorts.
• Mismatching grain to usage frequency — measuring a monthly-use product on daily retention manufactures a fake cliff.
• Trusting tiny or partial cohorts — recent cohorts have small N and incomplete periods; never call a trend off a cohort with 3 periods of data and N=12.
• Using a vanity return event (any login) instead of a value moment — it flatters the curve and hides real churn.
• Confusing the table's lower-right emptiness for churn — those cells are blank because those cohorts are too young to have reached that period, not because users left.

Tips

• The plateau, not the slope, is the prize. A curve that flattens at 30% beats one that starts at 60% and slides to zero. Find where it flattens.
• Anchor every cohort-over-cohort claim to a ship date. "P1 jumped after the new onboarding (released wk of [date])" is a story leadership can act on; a number alone isn't.
• Compare to a benchmark band, not zero. Know roughly what good looks like for your category (e.g. consumer social vs. B2B SaaS retain very differently) before declaring a curve "bad."
• One inflection, one action per pass. Pick the highest-leverage period, intervene there, and re-pull the table — don't try to fix the whole curve at once.