Glossary of Terms for Item Statistics D-Parm IRT d parameter A measure of the dispersion of the item computed using IRT method. Score_Conversion Score Conversion Table Raw scores and their corresponding converted values (e.g. the values to be used in score reports). AIS Average Item Score A group-dependent measure of item difficulty. Equal to the mean of the item scores for a group of individuals. For a rights-only dichotomously scored test, equivalent to the percent answering the item correctly (P-value). A-Parm IRT a parameter Index of item discrimination based on item response theory. B-Parm IRT b parameter Index of item difficulty based on item response theory. C-Parm IRT c parameter The lower asymptote of the item response function. Generally an index of guessing, i.e., the probability of getting the item correct for very low ability examinees. P-value P-value A group-dependent measure of item difficulty. It is equal to the percent answering the item correctly. PHI Phi-coefficient A group-dependent measure of item discrimination, Phi is the correlation of two dichotomous variables. In item analysis, it is generally the correlation between getting an item right or wrong and some dichotomous variable, e.g., pass, fail. rbis r-biserial A group-dependent measure of item discrimination, r-biserial is an estimate of the biserial correlation between a latent continuous variable hypothesized to underlie performance on a dichotomous item (i.e. item score) and a criterion which is a variable than can take many possible values, such as total score on the test. PTbis r-pointbiserial A group-dependent measure of item discrimination, r-pointbiserial is the point biserial correlation between the score on a dichotomous item and a criterion which is a variable than can take many possible values, such as total score on the test. Polyserial Polyserial The polyserial correlation is the generalization of the biserial correlation for use with either dichotomous or polytomous items. For example, it may be the generalized form of the correlation with a criterion. It is the correlation of the item score with some continuous variable, usually the total score on the test.