How do we estimate sigma2? what are two diagnostic checks, Ask an Expert

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In Ordinary Least Squares (OLS) Regression we assume that the response variable is normally distributed with mean XB and variance sigma^2, i.e. Y ~ N(XB, sigma^2).

a. How do we estimate sigma^2?

b. What are two diagnostic checks of model goodness-of-fit that we perform in order to assess this distributional assumption?

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