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Case Study A - Signup Conversion Dropped 8% Last Week

A worked case study: how to investigate a sudden metric drop. End with a written diagnosis (paste into the activity log to complete).

3 minutesReading
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The situation

It's Monday morning. The Slack channel for the growth team has 47 unread messages. The CEO posted at 7am:

"Why did signups drop 8% last week? Need an answer by EOD."

You're the analyst on point.

What you know

  • Weekly signups: usually ~10,000. Last week: 9,200.
  • The marketing team didn't change any campaigns.
  • Engineering didn't ship anything to the signup flow.
  • It was a normal week - no holiday, no outage anyone knows about.

Your job

Walk through the investigation step by step:

  1. Establish the magnitude (signal vs noise using trailing std).
  2. Decompose by source (channel shifts).
  3. Decompose by funnel step.
  4. Decompose by segment (device, geo, browser).
  5. Check for external events and tracking pixel regressions.
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