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The χ2 distribution is a special case of the gamma distribution where b = 2 in the equation for gamma distribution below.
The χ2 distribution gets special attention because of its importance in normal sampling theory. If a set of n observations is normally distributed with variance σ2, and s2 is the sample standard deviation, then
This relationship is used to calculate confidence intervals for the estimate of the normal parameter σ2 in the function normfit.