This Table identifies Differential Group Functioning DGF interactions between classification-groups of persons (identified by DIF=, referencing the person labels) and classification-groups of items (identified by DPF=, referencing the item labels) using the column selection rules. Differential average classification-group performance (DGF) is powerful when looking for latent classes among the persons. Differential bundle functioning (DBF) is powerful when looking for local dependence among the items. For more details, see Table 30 (DIF) and Table 31 (DPF). A graphing technique can be used to display DIF item characteristic curves for non-uniform DIF.

 

The log-odds model for an individual person in the group and item in the group is:

log(Pni1/Pni0) = Bgn - Dhi - Mgh

where

Bgn is the overall estimate from the main analysis of the ability of person n (who is in group DIF g)

Dhi is the overall estimate from the main analysis of the difficulty of item i (which is in DPF group h)

Mgh is the interaction (bias, DGF) for person group g on item group h. This is estimated from all persons in group g combined with all items in group h.

 

The DGF dialog displays when Table 33 is called from the Output Tables or Plots menus.

 

Table 33.2

 

DGF CLASS-LEVEL BIAS/INTERACTIONS FOR DIF=@GENDER AND DPF=$S1W1

---------------------------------------------------------------------------------------------

| ITEM    DGF    DGF   DGF  ITEM    DGF    DGF   DGF    DGF    JOINT     Welch       PERSON |

| CLASS SCORE   SIZE  S.E.  CLASS SCORE   SIZE  S.E. CONTRAST  S.E.   t  d.f. Prob.  CLASS  |

|-------------------------------------------------------------------------------------------|

| 1       .00    .06   .28  2       .01   -.18   .50      .24   .57   .42 119 .6774   F     |

| 1       .00    .06   .28  3       .04   -.50   .83      .56   .87   .64  30 .5261   F     |

| 1       .01   -.16   .27  2       .00    .00   .61     -.16   .67  -.24 100 .8097   M     |

| 1       .01   -.16   .27  3      -.04    .76  1.04     -.92  1.07  -.86  25 .3973   M     |

| 2       .01   -.18   .50  1       .00    .06   .28     -.24   .57  -.42 119 .6774   F     |

| 2       .01   -.18   .50  3       .04   -.50   .83      .32   .97   .33  39 .7420   F     |

 

This Table contrasts, for each item class, the size and significance of the Differential Item Functioning for pairs of person classifications.

 

The most important numbers in Table 33.2: The DGF CONTRAST is the difference in difficulty of the item between the two groups. This should be at least 0.5 logits for DGF to be noticeable. "Prob." shows the probability of observing this amount of contrast by chance, when there is no systematic item bias effect. For statistically significance DGF on an item, Prob. < .05.

 

DGF class specification defines the columns used to identify DGF classifications, using DIF= and DPF=, see the selection rules.

 

Reading across the Table 33.2 columns:

ITEM CLASS is the item classification specified by DPF=., e.g., the first here is CLASS is "1".

DGF SCORE is the average response score-point difference between the observed and the expected scores for this PERSON CLASS on this ITEM CLASS. Higher scores mean locally higher ability or locally lower difficulty.

DGF SIZE is the differential difficulty of this item for this class, with all else held constant, e.g., .06 is the relative difficulty for TAP Class "1" on KID Class "F". The more difficult, the higher the DGF measure.
-.52> reports that this measure corresponds to an extreme maximum person-class score. EXTRSCORE= controls extreme score estimate.
1.97< reports that this measure corresponds to an extreme minimum person-class score. EXTRSCORE= controls extreme score estimate.
-6.91E reports that this measure corresponds to an item with an extreme score, which cannot exhibit DIF>

DGF S.E. is the standard error of the DGF SIZE.

ITEM CLASS is the item classification specified by DPF=., e.g., the second CLASS is "2".

DGF SCORE is the average response score-point difference between the observed and the expected scores for this PERSON CLASS on this ITEM CLASS.

DGF SIZE is the differential difficulty of this item for this class, with all else held constant, e.g., -.15 is the relative difficulty for Kid Class M on Item Class1. The more difficult, the higher the DGF measure.

DGF S.E. is the standard error of the second DGF SIZE.

DGF CONTRAST is the difference between the DGF SIZEs, i.e., size of the DGF across the two classifications of persons, e.g., .07 - -.15 = .23 (usually in logits). A positive DGF contrast indicates that the item is more difficult for the first, left-hand-listed CLASS. See details in Table 33.3.

JOINT S.E. is the standard error of the DGF CONTRAST = sqrt(first DIF S.E.² + second DIF S.E.²), e.g., .38 = sqrt(.27² + .27²)

t gives the DGF significance as a Student's t-statistic = DGF CONTRAST / JOINT S.E. The t-test is a two-sided test for the difference between two means (i.e., the estimates) based on the standard error of the means (i.e., the standard error of the estimates). The null hypothesis is that the two estimates are the same, except for measurement error.

d.f. is the joint degrees of freedom. This is shown as the sum of the sizes of two classifications (see Table 33.3 less 2 for the two measure estimates, but this estimate of d.f. is somewhat high, so interpret the t-test conservatively, e.g., d.f. = (426 A + 1 D - 2) = 425. When the d.f. are large, the t statistic can be interpreted as a unit-normal deviate, i.e., z-score.

INF means "the degrees of freedom are so large they can be treated as infinite", i.e., the reported t-value is a unit normal deviate.

Prob. is the probability of the reported t with the reported d.f., but interpret this conservatively, i.e., "barely significant is probably not significant". If you wish to make a Bonferroni multiple-comparison correction, compare this Prob. with your chosen significance level, e.g., .05, divided by the number of DIF tests in this Table.

PERSON CLASS identifies the CLASS of persons specified with DIF=, e.g., the first here is CLASS is "F".

 

Each line in the Table is repeated with the ITEM CLASSes in reversed order.