{
  "nature": "original fictional teaching case",
  "window": "seven complete days after signup for each user",
  "activation": "create a timetable and invite at least one member",
  "return": "edit on a different day after activation, within the same seven-day signup window",
  "cohorts": [
    {"id": "A", "visitors": 1000, "signups": 100, "activated": 40, "returned": 16},
    {"id": "B", "visitors": 1000, "signups": 100, "activated": 25, "returned": 10}
  ],
  "unknowns": ["channel mix", "device mix", "activation timing", "observed errors", "interview evidence"]
}