Fannie Mae·Data Scientist·Technical Phone Screen
May 2026
Technical interview for a Data Scientist role at Fannie Mae that ended up being way more frontend/QA-architecture heavy than I expected. Two big parts: a conceptual breakdown of UI vs backend testing across a full test pyramid, then a deep-dive test matrix for a promo code feature. Not your typical data science interview.
- For a single-page app backed by a BFF that calls a payment API, how do you decide what belongs in UI tests versus backend tests, and which layers do you mock at each level of the test pyramid?
- How do you prevent flaky tests caused by network instability, timing issues, and async behavior, and how do you detect regressions using test oracles, golden images, or contract tests?
- Where do security checks like XSS, CSRF, and authorization validation belong in the test layers, and how do you enforce idempotency when retries happen?
- How do you measure test coverage beyond line or branch coverage, specifically around requirements coverage, risk coverage, and data coverage?
- Design a test matrix for a new promo code field on a checkout page. The code is validated asynchronously via the BFF, updates the order total, persists the promo, and fires an analytics event. Cover functional, boundary, accessibility, cross-compatibility, resilience, contract, and analytics scenarios.
- For the promo code feature, how do you test resilience scenarios like offline mode, server timeouts, 429 rate limits, and 5xx errors, and how do you ensure the UI eventually reflects server truth after stale state?
“This one sprawled fast.”