... distribution of χ2. Under the null hypothesis (and the conditions of this test), the distribution of χ2 is known as the chi-squared distribution.

A chi-squared distribution looks very different from a normal distribution, though the two are closely related (Behind the Math: Normal and Chi-Squared Distributions). A normal distribution is symmetric around its mean and assigns probabilities to both negative and positive values. In contrast, a chi-squared distribution is right-skewed and assigns probabilities only to positive values. Since χ2 sums squared deviations, it can never be negative. Hence, the chi-squared distribution has a lower limit ...

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