From the Plots menu, DPF Plot 31 produces the plotted output corresponding to Table 31.2. It shows differential person functioning (DPF).
From the Output Tables menu, the DPF dialog is displayed.
In the DPF= box specify the column in the item label that identifies the DPF classification for each person.
Select what data point labels are to be displayed on the Excel plot. In this case, the person label. The item classifications will be identified by their column codes.
There are three standard plots:
DPF Measure (abil.) |
reports the ability of the person for each item classification: DPF "local absolute" Measure = DPF "relative" Size + Overall "baseline" person ability |
DPF Size (abil.) |
reports the size of the person DPF for the item classification relative to the overall "baseline" person ability |
t-value (abil) |
reports a simple approximate t-test of the person DPF against overall ability. The t-statistic is the statistical probability of the DPF size relative to its measurement error expressed as an approximate unit-normal deviate. A critical values is usually ±2. This tests the hypothesis that the DPF size can be attributed to measurement error. |
If the person classifying variable is conceptually smooth, such as "age", then smoothing the lines in this plot makes sense:
In your Excel chart, right-click on the data series that you want to smooth.
Excel displays a Context menu.
Choose Format Data Series from the Context menu.
Excel displays the Format Data Series dialog box.
Make sure the Patterns tab is displayed.
Select the Smoothed Line check box.
Click on OK.