Modeling and Multivariate Methods - SAS

Chapter 22 Scoring Tests Using Item Response Theory 533
Platform Commands
Figure 22.12 Parameter Estimates
Item
identifies the test item.
Difficulty is the b parameter from the model. A histogram of the difficulty parameters is shown beside the
difficulty estimates.
Discrimination is the a parameter from the model, shown only for 2PL and 3PL models. A histogram of
the discrimination parameters is shown beside the discrimination estimates.
Threshold
is the c parameter from the model, shown only for 3PL models.
Platform Commands
The following three commands are available from the drop-down menu on the title bar of the report.
Number of Plots Across brings up a dialog to specify how many plots should be grouped together on a
single line. Initially, plots are stacked one-across. Figure 22.8 on page 530 shows four plots across.
Save Ability Formula creates a new column in the data table containing a formula for calculating ability
levels. Since the ability levels are stored as a formula, you can add rows to the data table and have them
scored using the stored ability estimates. In addition, you can run several models and store several
estimates of ability in the same data table.
The ability is computed using the IRT Ability function. The function has the following form
IRT Ability (Q1, Q2,...,Qn, [a1, a2,..., an, b1, b2,..., bn, c1, c2, ...,
cn]);
where Q1, Q2,...,Qn are columns from the data table containing items, a1, a2,..., an are the
corresponding discrimination parameters, b1, b2,..., bn are the corresponding difficulty
parameters for the items, and c1, c2, ..., cn are the corresponding threshold parameters. Note that
the parameters are entered as a matrix, surrounded by square brackets.
Script
contains options that are available to all platforms. See Using JMP.