Diagnose a single-day drop in Facebook birthday greetings

You are the product manager for Birthdays on Facebook: the reminders that tell people a friend has a birthday and the places where they can post or send a greeting. On one particular day, the number of birthday greetings drops. Walk through how you would find out why it happened and decide what to do. ```hint Use the shape of the drop A fall confined to a single day points to different causes than a slow slide. Pin down what that day would normally have looked like, then break the greeting count into pieces you can check one at a time. ``` ### Clarifying Questions - What counts as a greeting: a post on the person's profile, a comment, a message sent from a birthday reminder, or all of these? - How large is the drop, and against which baseline: the day before, the same weekday in recent weeks, or the same date last year? - Is it spread across every country, platform and app version, or concentrated in a few of them? - Did the number recover the next day, or is that day the start of a lower level? ### What a Strong Answer Covers - Checking that the drop is real: logging and pipeline health, metric definition changes, and a baseline that respects weekday and calendar effects. - A decomposition of greetings into birthdays that day, friends reminded, reminders seen, share who greet and greetings per greeter, used to locate where the loss happened. - Segmentation by platform, app version, region and entry point to separate a release or outage from a behavior change. - Internal and external hypotheses checked in a sensible order, each with the data that would confirm or rule it out. - A response tied to the cause (roll back, fix, accept, or redefine the metric) and a way to catch the next drop sooner. ### Follow-up Questions - The whole drop is on Android, in the newest app version. What do you do in the next hour? - Greetings posted on profiles fell, but birthday messages sent privately rose by about the same amount. Is that a problem? - How would you set up alerting so a drop like this is noticed the same day rather than the next week?