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The Poisson distribution
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Learn it For events happening at an average rate λ in a fixed interval (calls per hour, typos per page): P(X = k) = e⁻λ λᵏ/k!. Mean and variance both equal λ. Rates scale: 3 per hour means 6 per two hours.
Example: A help desk gets 4 calls an hour on average. P(exactly 2 in an hour)?
- e⁻⁴ × 4²/2!
- ≈ 0.147.
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