chi-square
(symbol: χ2) n. a statistic that is the sum of the squared differences between the observed scores in a data set and the expected value. That is, to obtain the chi-square one finds the difference between each observed score and the expected score, squares that difference, and divides by the expected score; finally, one adds the resulting values for each score in the set. The smaller the chi-square, the more likely it is that the model from which the expected score is obtained provides a legitimate representation of the phenomenon being measured.