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What is Chi-square? History and Definition . A chi-square distribution is the distribution of the sum of squares of k independent standard normal random variables with k degree of freedom. A chi-square test is a statistical hypothesis test where the null hypothesis that the distribution of the test statistic is a chi-square distribution, is true. While the chi-square distribution was first introduced by German statistician Friedrich Robert Helmert, the chi-square test was first used by Karl Pearson in 1900.
The use of a chi-square distribution also requires the use of degrees of freedom. Here, in an identical manner as with the t distribution, the sample size determines which distribution to use. If the sample size is n, then there are n - 1 degrees of freedom.