Brings up a dialog to specify how many plots should be grouped together on a single line. Initially, plots are stacked one-across. shows four plots across.
Creates a new column in the data table containing a formula for calculating ability levels. Because 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
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, enclosed in square brackets.