Compute a running mean and variance over a stream in one pass (Welford's algorithm). Why not the naive formula?
The textbook variance formula suffers catastrophic precision loss on streaming data and cannot update incrementally. Welford's one-pass update is the norm for online feature normalization. Here it is.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
The textbook variance formula suffers catastrophic precision loss on streaming data and cannot update incrementally. Welford's one-pass update is the norm for online feature normalization. Here it is.
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.
