Table 31.3 Differential person functioning DPF list
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Table 31 supports the investigation of item bias, Differential Person Functioning (DPF), i.e., interactions between individual persons types of items. Specify DPF= for item classifying indicators in item labels. Person bias and DPF are the same thing.
You want to examine person bias (DPF) between starting-blocks in Exam1.txt. You need a column in your Winsteps item label that has two (or more) item type codes.
DPF class specification is: DPF=$S1W2
| TAP OBSERVATIONS BASELINE DPF DPF DPF DPF DPF KID |
| CLASS COUNT AVERAGE EXPECT MEASURE SCORE MEASURE SIZE S.E. t Prob. Number Name |
| 1- 11 .18 .23 -2.94 -.05 -3.52 -.58 1.05 -.55 .5937 1 Adam M|
| 1- 11 .36 .39 -.26 -.03 -.67 -.41 1.35 -.30 .7686 2 Anne F|
This displays a list of the local difficulty/ability estimates underlying the paired DPF analysis. These can be plotted directly from the Plots menu.
The DPF effects are shown ordered by CLASS within person (row of the data matrix).
TAP CLASS identifies the CLASS of items. KID is specified with ITEM=, e.g., the first CLASS is "1-"
OBSERVATIONS are what are seen in the data
COUNT is the number of observations of the classification used for DPF estimation, e.g., 11 "1-" items responses were made by person 1.
AVERAGE is the average observation on the classification, e.g., 0.18 is the average score class "1-' items by person 1.
COUNT * AVERAGE = total score of person on the item class.
BASELINE is the prediction without DPF
EXPECT is the expected value of the average observation when there is no DPF, e.g., 0.92 is the expected average for person 1 on item class "1-" without DPF.
MEASURE is the what the overall ABILITY measure would be without DPF, e.g., -2.94 is the overall person ability of person 1 as reported in Table 18.
DPF: Differential Person Functioning
DPF SCORE is the difference between the observed and the expected average observations, e.g., 0.92 - 0.89= -0.03
DPF MEASURE is the person ability for this item class, e.g., person 1 has a local ability of -3.52 for item CLASS "1-".
The average of DPF measures across CLASS for an item is not the BASELINE MEASURE because score-to-measure conversion is non-linear. ">" (maximum score), "<" (minimum score) indicate measures corresponding to extreme scores.
DPF SIZE is the difference between the DPF MEASURE for this class and the BASELINE measure ability, i.e., -3.93 - -4.40 = .48. Item 4 is .48 logits more difficult for class F than expected.
DPF S.E. is the approximate standard error of the difference, e.g., 0.89 logits
DPF t is an approximate Student's t-statistic test, estimated as DPF SIZE divided by the DPF S.E. with a little less than (COUNT-2) degrees of freedom.
Prob. is the probability of the t-value. This is approximate because of dependencies between the statistics underlying the computation.
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