Reddit · Metrics · Mid · 45 to 60 minutes

Why did Reddit data transfer triple overnight?

Reddit is an online platform where people join communities to post, comment, vote, and consume text, image, and video content. Answer this product management case and get it graded line by line against a weighted rubric.

Root cause analysis
Metric judgment
Prioritization
Technical fluency
Stakeholder communication

The situation

You are a PM on Reddit's core browsing experience team. On Monday morning, the engineering manager for performance and a finance partner both escalate the same issue: Assume a dashboard now shows bytes transferred per daily active user up about 3x since the previous night. If the spike is real, Reddit could face higher infrastructure cost, slower experiences for users on weaker connections, and pressure from leadership to pause upcoming launches until the cause is clear. If the spike is not real, the company could waste time on false alarms and make bad product calls. There is no obvious sitewide outage, traffic looks roughly normal at a high level, and several teams shipped small changes last week across feed, media, and analytics. You are not expected to debug code yourself, but you do need to decide how Reddit should investigate over the next 24 to 48 hours, what hypotheses to prioritize, who to involve, and whether any immediate product or release actions are warranted. The hard part is separating a real user behavior shift from a measurement or instrumentation artifact quickly and credibly.

What to produce

Write a 1 page investigation memo, 600 to 800 words.

Your answer must

State the decision you are trying to support in the next 24 to 48 hours, not just a list of possible causes.
Lay out a step by step root cause analysis plan that starts with validating the metric and instrumentation before moving to other cause categories.
Identify the key slices or cuts of data you would check first to isolate where the spike is coming from, such as platform, app version, surface, media type, geography, or release timing.
Provide a prioritized hypothesis list covering measurement issues, technical regressions, product changes, and external behavior or traffic shifts, with evidence you would seek for each.
Recommend any immediate actions, escalation thresholds, and stakeholder communications for engineering, leadership, and finance while the investigation is in progress.

Constraints

Use only existing dashboards, logs, release notes, and known team contacts in the first day, do not assume new experiments or new instrumentation can be created immediately.
You have limited engineering time, at most one engineer each from performance, analytics, and media infrastructure can help during the first 24 hours.
Do not recommend a rollback, launch freeze, or broad incident declaration unless your plan explains what evidence would justify it.

Format

Use short sections with bullets where helpful. End with a ranked next step list for the first 24 hours.

How Reddit actually played it

Teaches root cause analysis for a sudden metric movement, starting from the author's own story of a data transfer metric jumping 3x overnight at Google. It then walks through checking data quality, technical issues, product changes and external causes in order.

Verify the data and instrumentation before hunting for causes
Eliminate categories of causes systematically
Distinguish a real behavior change from a measurement artifact

Built from Product Execution Question: Reddit Root Cause Analysis Case Study by Ajitesh, Tough Tongue AI (PM Interview Prep Weekly).