Walk me through instrumental variables: what makes an instrument valid, and how do two-stage least squares and LATE work?
IV is the causal tool people name but cannot defend. The signal is spelling out the two conditions an instrument must meet, why one is testable and one is not, and what 2SLS actually estimates. Here is the answer.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
IV is the causal tool people name but cannot defend. The signal is spelling out the two conditions an instrument must meet, why one is testable and one is not, and what 2SLS actually estimates. Here is the answer.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
