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.