Evaluate impact of short videos in feed

Scenario

You work on the core News Feed of a social app. The team is planning to roll out a short-form video module called "Reels" inside the feed.

Prompt

1. How would you assess whether adding Reels to the News Feed is successful?
2. What metrics would you choose (primary + secondary/guardrails)?
3. How would you set up an experiment / launch plan to measure the impact?

Notes / Assumptions

• The feed already carries mixed content types: text, images, long videos, and links.
• Reels could affect both consumption (viewing) and distribution (other content receiving less exposure).
• You have standard event logging for impressions, clicks, watch time, likes, comments, shares, hides, follows, and so on.

Overview: This prompt tests skills in metric definition, causal experimentation, product analytics, and feed-system behavior in the System Design and Data Engineering area.

See the full data-engineer interview account from which this question came.