In Excel 2010, the CHIINV() function was replaced with the CHISQ.INV.RT() function, and the CHISQ.INV() function was added to increase the accuracy of the results. To ensure the backward compatibility of CHISQ.INV.RT(), the CHIINV() function is still available.
Definition. This function returns the inverse of the right-tailed probability of the chi-square distribution. If probability = CHISQ.DIST.RT(x,...) then CHI.INV(probability,...) = x. Use this function to compare the observed results with the expected results in order to decide whether your original hypothesis is valid.
probability (required). A probability associated with the χ2-distribution