Interview Questions › Root Cause Analysis

Average session duration is down, but retention is stable

Average session duration on YouTube mobile decreased by 18% over the past month, yet D1, D7, and D30 user retention remained completely rock solid. How do you explain and evaluate this?

Context

Total daily active users and app opens per user actually increased by 10%.

Clarifying questions to ask first

• Did total daily watch time across all sessions go down, or just duration per individual session?
• Did session frequency (app opens) increase?

How to approach it

1. Analyze format consumption mix shifts (Shorts vs Long-form videos vs Live streams)
2. Evaluate whether shorter sessions represent higher user efficiency or shallower engagement
3. Segment by User Age Cohort, Device Type, and Time of Day (Commute vs Evening lean-back)
4. Determine if overall watch time (Sessions × Duration) is net positive or negative

Framework: Metric Interdependence (Sessions × Duration = Total Time) -> Format Mix Shift (Shorts vs Longform) -> User Intent & Cannibalization -> Net Business Impact

Sample answer

• YouTube Shorts algorithmic push resulting in more frequent but shorter micro-sessions throughout the day
• Video player loading faster, reducing idle waiting time
• UI change making it easier to find quick answers (tutorials/how-to)

1. App opens per user increased from 3.2 to 4.8 per day; Shorts consumption surged 60%
2. Total daily watch time per user is actually up 4% despite lower per-session duration

Healthy format shift: users open the app more frequently for quick Shorts sessions; total watch time and ad engagement grew overall.

What interviewers look for

• Total Watch Hours
• Sessions per User / Day
• Shorts Views vs Long-form Views
• Ad Impressions per User