I believe the correct word to fill in the blank would be "chance". A chi-square test involves a comparison between what is observed and what would be expected by chance. It tests the the observed values and the values that are expected theoretically. The chi-square test is a statistical tool where the distribution of sampling is considered as a chi squared distribution when the null hypothesis is found to be true. There are two types of this test namely the chi-square goodness of fit test and chi-square test for independence. The former type tests a data whether it matches the population while the latter assess two variables using a contingency table testing if a relation is present.