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Compute a running mean and variance over a stream in one pass (Welford's algorithm). Why not the naive formula?

The textbook variance formula loses catastrophic precision on streaming data and cannot update incrementally. Welford's one-pass update is the standard for online feature normalization. Here it is.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

The textbook variance formula loses catastrophic precision on streaming data and cannot update incrementally. Welford's one-pass update is the standard for online feature normalization. Here it is.

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