Experimentation
189 bites tagged Experimentation — interview questions with model answers, and 60-second explainers.
Propose a multi-armed bandit system to optimize headlines faster
This tests online learning and the explore-exploit tradeoff. Answers contrast fixed A/B with adaptive allocation, sketch a Bayesian bandit service with a min exploration rate, and note delay.
How would you A/B test sign-up button copy and measure results?
This tests basic experimental design. A strong answer covers: random assignment, serving variant copy, tracking impressions and conversions, and measuring lift. A red flag is sequential testing or vanity metrics like clicks without sign-ups.
Explain how CUPED increases statistical power and required data
Tests ANCOVA variance reduction. Answer: CUPED regresses pre-experiment X on Y, shrinking variance by (1-ρ²); needs pre-randomization prognostic baseline; beats difference scores. Red flag: calling it Y-X subtraction or saying it changes the effect.
What is the difference between a primary metric and a guardrail metric?
Tests whether you distinguish success criteria from safety checks in experiments. A strong answer defines primary metrics as the target outcome, guardrails as protective thresholds, and gives a concrete scenario where a primary lift does not justify shipping…
What is the 'novelty effect' in experimentation?
Tests whether you separate temporary curiosity from durable value. A strong answer defines novelty effect as short-term behavior change triggered by new elements, notes it inflates early experiment lift, and proposes longer runtimes or lagged cohort analysis.
Why is stopping an A/B test at first significance problematic?
Tests peeking and Type I error inflation. Name peeking; explain daily looks inflate false positive rates above nominal alpha; note p-values assume one look at fixed sample size; recommend pre-committed runtimes or sequential testing.
How do you determine sample size and duration for an A/B test?
This tests statistical power literacy. A strong answer names baseline rate, MDE, alpha, and beta; explains the duration versus sensitivity trade-off; and notes traffic allocation. A red flag is ignoring power or stopping early when results look significant.
Design an A/B test for a 'Buy Now' button color change
Tests structured experiment design from hypothesis to metric. Strong answers: define a falsifiable hypothesis; pick purchase conversion as primary; size the sample and duration; randomize by user; pre-commit to stopping rules.
Explain Simpson's Paradox and construct a user engagement scenario
Tests whether you spot trends reversing when population mixes differ. Good answers define the paradox, give a numerical example with per-segment wins but aggregate loss, and warn against segment-only decisions.
A/B test p-value 0.08, PM wants to ship. How do you advise?
Tests statistical rigor versus business pragmatism. A strong answer covers pre-registered thresholds, false positive risk, statistical power, confidence intervals, and the business cost of being wrong. Red flag: shipping without quantifying downside risk.
A/B test shows 5% lift with imbalanced mobile traffic. What next?
This tests sample ratio mismatch and Simpson's paradox. A strong answer says the lift is confounded by platform, demands stratified analysis to isolate the true effect, and checks randomization. A red flag is accepting the 5% lift without questioning skew.
How would you determine if Feature X causally drives higher retention?
Tests causal inference intuition for product metrics. Great answers propose a randomized holdback or instrumental variable, control for user intent, and estimate a local average treatment effect.
How do you diagnose why a new feature's adoption is flat?
Tests structured analytics thinking across the adoption funnel. A strong answer maps discovery to habituation, segments cohorts, pairs behavior with feedback, and validates via experiments. Red flag: blaming UI without proving users know the feature exists.
When is an A/B test not feasible, and what is DiD?
This tests your grasp of causal inference when randomization isn't possible. Explain a scenario like a state-level launch, introduce Difference-in-Differences (DiD), and state its core parallel trends assumption.
How do you handle the multiple comparisons problem in A/B testing?
Tests your grasp of statistical risk in experimentation. Explain how multiple tests inflate false positives, then describe mitigations like Bonferroni correction or limiting concurrent tests. A red flag is suggesting total test isolation, which is impractical.
How CUPED increases statistical power in experiments
Tests your grasp of variance reduction in A/B testing. Explain how CUPED uses correlated pre-experiment data to reduce outcome variance, increasing statistical power. A red flag is confusing it with simpler difference scores, which can actually increase noise.
Why is user-level randomization flawed by spillover effects?
Tests your grasp of SUTVA violations in network experiments. Explain how spillover contaminates the control group, then propose graph cluster randomization—grouping users and assigning entire clusters to A/B variants—to minimize interference.
What is the novelty effect in experimentation?
This tests your grasp of temporary user behavior changes that can invalidate A/B tests. A strong answer defines the effect, explains how it inflates metrics, and suggests running tests longer or segmenting by user tenure. A red flag is ignoring mitigation.
How would you A/B test a 'Buy Now' button color change?
Tests your grasp of the A/B testing lifecycle. A strong answer defines a clear hypothesis (e.g., 'a green button will increase clicks'), selects a primary metric (CTR), and considers guardrail metrics. A red flag is skipping the hypothesis and metrics.
Handling spillover effects in social network A/B tests
This tests your grasp of SUTVA violations in networked experiments. A great answer explains how user-level randomization causes spillover, then proposes graph cluster randomization to assign entire communities to treatment or control, minimizing…
Handling the novelty effect in experimentation
This tests your grasp of second-order effects in A/B testing. A great answer defines the novelty effect, explains how it inflates initial metrics, and suggests mitigating it by running tests longer or segmenting by user tenure. A red flag is ignoring it.
How would you A/B test a 'Buy Now' button color change?
This tests structured thinking. A good answer defines a hypothesis, selects primary and guardrail metrics, and outlines the experiment's duration and analysis plan. A red flag is focusing only on clicks without considering business impact.
Randomized Controlled Trials: The Gold Standard for 'Did It Work?'
An RCT is a science fair experiment for business decisions, isolating one change to see its true effect. It's used in A/B tests to prove a new feature worked. The biggest footgun is peeking at results early, which can lead to false conclusions.
Quantitative Growth Model: The Spreadsheet That Runs Your Business
A quantitative growth model is a spreadsheet that maps your business, showing how inputs like ad spend turn into revenue. It's used to forecast growth, simulate strategy changes, and set goals. The footgun: your model is only as good as its assumptions.
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