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How can I calculate the Standard deviation of my estimate for the river B, as my estimation is B = C - A, and C and A correlate negatively (-0.5)?

kjetil b halvorsen
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Severin Spörri
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    Convert the correlation to a covariance and apply [the formula for a sum of covariances](https://stats.stackexchange.com/questions/346327/intuition-behind-the-formula-for-the-variance-of-a-sum-of-two-variables). – whuber Aug 25 '21 at 16:29
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    σ^2(x-y) = σ^2(x)+σ^2(y)-2*ρ*σ(x)*σ(y). σ^2(x-y) = (25 + 25 - 2 * -0.5 * 5 * 5 ) L^2 / s^2 =75 L^2 / s^2, SD = 8.66 L / s (just like in the solution) Thank you!! – Severin Spörri Aug 25 '21 at 17:26

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