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Explain stationarity in a time series
This tests your grasp of core time series modeling assumptions. A strong answer defines stationarity (constant mean/variance), explains its importance for ARIMA (stable patterns), and names a test (ADF) and a fix (differencing).
Random split vs walk-forward validation in forecasting
Random splits leak future data into training; walk-forward validation rolls the origin ahead, testing only on later observations.
Train-Test Split vs. Time-Series Cross-Validation
This tests your grasp of data leakage in temporal data. A good answer explains why random splits create lookahead bias, then details how rolling-origin validation respects time. A red flag is just describing methods without explaining *why* one is necessary.
Train-test split vs. time-series cross-validation?
Tests if you see why temporal data breaks random splits. Contrast random sampling with sequential 'walk-forward' validation, where you only use past data to predict the future.
Which classical baseline model handles weekly seasonality and upward trend?
Tests matching model structure to data characteristics. Name Holt-Winters triple exponential smoothing; map its level, trend, and seasonal equations to weekly period. Red flag: jumping to SARIMA without explaining why ETS is the natural baseline.
Which model for forecasting with seasonality and trend?
This tests your knowledge of classical time series models. A good answer names Holt-Winters, explaining its level, trend, and seasonal components. It also discusses choosing between additive and multiplicative methods. A red flag is jumping to complex models.
Forecasting inventory with trend and weekly seasonality?
This tests mapping a business problem to a statistical tool. A good answer names Holt-Winters, explains its level, trend, and seasonal components, and discusses additive vs. multiplicative seasonality.
Diagnosing model degradation over time
Name it model drift, split data vs concept drift; diagnose by comparing distributions and ruling out pipeline bugs; fix via monitoring and retraining.
How do you diagnose and fix a model's degrading performance?
Tests your MLOps process for handling model decay. Name it "concept drift," then outline a plan: diagnose by comparing data distributions, solve with a targeted retraining strategy, and implement proactive monitoring. A red flag is just saying "retrain it."
How do you handle model performance degradation over time?
This tests MLOps lifecycle awareness. Name concept drift, outline a systematic diagnosis of data and error patterns, discuss retraining strategies, and propose a monitoring plan. A red flag is just saying 'retrain the model' without any diagnosis.
Describe two methods for generating prediction intervals or probabilistic forecasts
Tests uncertainty quantification for risk-adjusted decisions. Strong answers: (1) parametric intervals via forecast error variance and normal multipliers, (2) bootstrap residual resampling for empirical percentiles.
Describe two methods for generating prediction intervals
This tests your grasp of uncertainty quantification. A great answer contrasts an analytical method (assuming normal errors, using multipliers like 1.96 for 95%) with a simulation method (bootstrapping residuals).
Describe two methods for generating prediction intervals
This tests your understanding of forecast uncertainty. Describe two methods: 1) assuming normally distributed errors and using a standard deviation multiplier, and 2) bootstrapping residuals to simulate future paths.
What event and data payload track Add to Cart actions?
This tests GA4 ecommerce schema design. Fire add_to_cart with items array containing item_id, price, currency, quantity; include user_id, user_segment, session_id, and timestamp. Red flag: generic button_click with DOM selectors instead of semantic data.
Describe the client-side event for an 'Add to Cart' button
Tests your knowledge of standard analytics schemas (like GA4) and designing payloads for business analysis. A great answer names the 'add_to_cart' event, details the 'items' array with product data, and mentions user/session context.
Describe the client-side event for an 'Add to Cart' button
This tests your ability to design analytics events for future analysis. Name a standard event like add_to_cart and list item parameters (item_id, price, quantity).
How would you design UTM capture and attribution persistence?
Capture UTMs on landing, store in a first-party cookie with TTL, attach to events, and persist on conversion.
How would you capture UTM parameters for attribution?
This tests your grasp of the data lifecycle from capture to persistence. A good answer covers client-side parsing, cookie storage, and linking anonymous data to a user record upon sign-up. A red flag is forgetting to persist the data server-side.
How would you capture and persist UTM parameters for attribution?
Tests your grasp of state management and data persistence for analytics. A good answer covers capturing UTMs with JS, persisting them in a cookie, and associating them with a user record on the server during a conversion event.
How would you instrument and query P95 API latency by region?
This tests white-box latency instrumentation and safe cardinality for percentile aggregation. Strong answer: emit histograms by region, query P95 with histogram_quantile or a log percentile, and keep trace IDs in logs only.