# Test Statistic in Chi-Square Test

Can anyone come up with an intuitive explanation or point me to a link that gives a derivation of the test statistic in chi-square test? I am having problems understanding why the particular test statistic is approximated by a Chi-Square random variable under the null hypothesis of the chi-square test. I cannot find any helpful literature online or in my textbooks too.

After all, the chi-square distribution with k degrees of freedom arises from the sum of squares of k standard normal distributions. This implies that for each bin, (observed-expected)^2 /expected ~ chi-square(1). Why?

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