Binomial distribution P(X=r) = ⁿCᵣ pʳ qⁿ⁻ʳ (P(X=r) = ⁿCᵣ · pʳ · qⁿ⁻ʳ where q=1−p, r=0,1,,n)

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P(X=r) = ⁿCᵣ · pʳ · qⁿ⁻ʳ where q=1−p, r=0,1,...,n

What each symbol means

SymbolWhat it stands for
nNumber of independent trials
pProbability of success in each trial
q1−p (probability of failure)
rNumber of successes

When to use this

Trials are independent, identical, with only two outcomes (success/failure). Mean = np, Variance = npq.