Data Science interview questions and answers

A box plot compactly compares medians, IQRs, and flagged extremes across groups, which is handy for many categories.

Heatmaps work well for correlation matrices, confusion matrices, or intensity over two discrete axes such as hour-by-weekday.

Lines preserve temporal order and make trends, seasonality, and shocks easy to see.

A confounder is a variable that influences both the treatment-like factor and the outcome, opening a back-door association.

Simpson's paradox is when a trend that appears in several groups reverses or vanishes when the groups are combined.

Reverse causality means the outcome is driving the supposed cause, not the other way around.

Stratified sampling draws separately within subgroups, such as country or plan tier, often in proportion to size.

Cluster sampling selects groups first, such as cities or schools, then observes units inside those groups.

The bootstrap resamples the observed dataset with replacement to approximate the sampling distribution of a statistic.

Sampling error is random fluctuation from drawing a finite sample, and it shrinks with n under good design.

Poisson models counts of events in a fixed interval when events are rare, independent, and have a constant average rate.

Binomial models the number of successes in a fixed number of independent Bernoulli trials with constant probability.

Products of many positive random shocks, or processes with multiplicative growth, tend toward lognormal.

Kurtosis describes tail heaviness and peak relative to a normal curve, with excess kurtosis often reported so normal equals zero.

A stationary series has a stable mean, variance, and autocovariance structure over time.

Lag features are past values of the series or of covariates, such as yesterday's demand or last week's price.

A rolling mean averages the last k points to smooth noise and reveal slower structure.

Random splits leak future information into training and overstate accuracy.

Retention is the share of a starting cohort that is still active after a stated window, such as day-7 return.

DAU and MAU count unique active users in a day or calendar month under a defined activity event.