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Variance and Standard Deviation in Probability

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Discrete Random Variable with Finite Outcomes

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Variance (σ2)(\sigma^2) is the sum of the product of each outcome's probability and the square of its deviation from the mean. σ=σ2\sigma = \sqrt{\sigma^2}

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Continuous Random Variable with Probability Density Function (PDF)

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σ2=(xμ)2f(x)dx\sigma^2 = \int_{-\infty}^{\infty} (x - \mu)^2 f(x) dx
Where f(x)f(x) is the PDF and μ\mu is the mean. σ=σ2\sigma = \sqrt{\sigma^2}

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Binomial Random Variable (n independent trials, each with success probability p)

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σ2=np(1p)\sigma^2 = np(1-p)
where nn is the number of trials and pp is the success probability. σ=np(1p)\sigma = \sqrt{np(1-p)}

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Poisson Random Variable (events in fixed interval with mean rate λ\lambda)

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Variance (σ2)(\sigma^2) is equal to the mean rate λ\lambda. σ=λ\sigma = \sqrt{\lambda}

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Uniform Random Variable (continuous, with range aa to bb)

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σ2=(ba)212\sigma^2 = \frac{(b - a)^2}{12}
where aa and bb are the minimum and maximum values, respectively. σ=(ba)212\sigma = \sqrt{\frac{(b - a)^2}{12}}

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Exponential Random Variable (time until event with rate λ\lambda)

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Variance (σ2)(\sigma^2) is given by 1λ2\frac{1}{\lambda^2}. σ=1λ\sigma = \frac{1}{\lambda}

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Normal Random Variable (mean μ\mu and variance σ2\sigma^2)

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Variance is given by parameter σ2\sigma^2. Standard Deviation is σ\sigma

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Geometric Random Variable (number of trials until first success, with success probability p)

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σ2=1pp2\sigma^2 = \frac{1-p}{p^2}
Standard Deviation σ=1pp2\sigma = \sqrt{\frac{1-p}{p^2}}

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Hypergeometric Random Variable (successes in samples without replacement)

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σ2=NKN1KNNnNn\sigma^2 = \frac{N-K}{N-1} * \frac{K}{N} * \frac{N-n}{N} * n
where NN is the population size, KK is the number of success states in the population, and nn is the sample size.

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Negative Binomial Random Variable (number of trials until r-th success)

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σ2=r(1p)p2\sigma^2 = \frac{r(1-p)}{p^2}
where rr is the number of required successes and pp is the probability of success on each trial. Standard Deviation σ=r(1p)p2\sigma = \sqrt{\frac{r(1-p)}{p^2}}

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Multinomial Random Variable (n independent trials, each with k possible outcomes)

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For any two outcomes ii and jj, Cov(Xi,Xj)=npipjCov(X_i, X_j) = -np_ip_j if iji \neq j. For outcome ii, Var(Xi)=npi(1pi)Var(X_i) = np_i(1-p_i). Standard Deviation σi=npi(1pi)\sigma_i = \sqrt{np_i(1-p_i)}

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Chi-Squared Random Variable (sum of squares of k standard normal random variables)

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Variance is 2k2k where kk is the degrees of freedom. Standard Deviation σ=2k\sigma = \sqrt{2k}

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