PayPal·Data Scientist·Technical Phone Screen
Jul 2026
PayPal data scientist round focused entirely on a streaming anomaly detection problem. Pretty deep systems-thinking question for a DS role, felt more like a software engineering interview at points.
- Design and implement a streaming login event processor that fires an alert whenever a user logs in from 4 or more distinct devices within any 10-minute sliding window. The function consumes a time-ordered stream of (user_id, timestamp, device_id, success) events. Walk through your data structures, eviction strategy, how you handle late-arriving events (up to 30 seconds), and the exact alerting semantics at window boundaries. Provide pseudocode.
“This was the whole interview basically.”