The statistic Chi Square () is what statisticians call an enumeration statistic. Rather than measuring the value of each of a set of items, a calculated value of Chi Square compares the frequencies of various kinds (or categories) of items in a random sample to the frequencies that are expected if the population frequencies are as hypothesized by the investigator.
Chi Square is often used to assess the "goodness of fit" between an obtained set of frequencies in a random sample and what is expected under a given statistical hypothesis. For example, Chi Square can be used to determine if there is reason to reject the statistical hypothesis that the frequencies in a random sample are as expected when the items are from a normal distribution.
In the equation shown below, O and E are Observed and Expected frequencies in each category.