Mae CHIDIST yn cymharu' r gwerth ail isradd Chi a roddir ar gyfer sampl hap - a gyfrifir o' r swm o ([gwerth a arsylwir - gwerth a ddisgwylir] ^2/ gwerth a ddisgwylir) ar gyfer pob gwerth - efo' r dosraniad ail isradd Chi damcaniaethol. Mae' n penodi tebygolrwydd gwall ar gyfer y rhagdybiaeth a arbrofir.
CHIDIST compares the Chi square value to be given for a random sample that is calculated from the sum of (observed value-expected value)^2/ expected value for all values with the theoretical Chi square distribution and determines from this the probability of error for the hypothesis to be tested.