DIF for two person groups

You have two person-groups in your DIF analysis. For your two groups on item 1.

 

Table 30.1

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

| PERSON   DIF   DIF   PERSON   DIF   DIF      DIF    JOINT  Rasch-Welch     MantelHanzl ITEM   

| CLASS  MEASURE S.E.  CLASS  MEASURE S.E.  CONTRAST  S.E.   t  d.f. Prob. Prob.  Size Number

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

| 1        -.47   .16  2       -1.57   .51      1.10   .53  2.07  62 .0422 .1986 +.         1

 

The DIF measure (item difficulty on the latent variable) for  Person group 1 on item 1 is -.47 logits

The DIF measure (item difficulty on the latent variable) for Person group 2 on item 1 is -1.57 logits

 

DIF Contrast = difference between item difficulties for the two groups = -.47 - - 1.57 = 1.10 logits

 

Table 14.1

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

|ENTRY   TOTAL  TOTAL           MODEL|   INFIT  |  OUTFIT  |PT-MEASURE |EXACT MATCH|

|NUMBER  SCORE  COUNT  MEASURE  S.E. |MNSQ  ZSTD|MNSQ  ZSTD|CORR.  EXP.| OBS%  EXP%|

|------------------------------------+----------+----------+-----------+-----------+

|     1    394     83    -.76     .16|1.07    .3| .77   -.3|  .27   .24| 81.9  82.1| 

 

The baseline difficulty (from Table 14.1) is -0.76 logits.

 

Table 30.2

 

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

| PERSON     OBSERVATIONS    BASELINE       DIF     DIF     DIF   DIF  DIF        ITEM

| CLASS     COUNT AVERAGE EXPECT MEASURE   SCORE MEASURE   SIZE  S.E.   t   Prob. Number

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

| 1            42    4.57   4.77    -.76    -.19    -.47    .29   .16  1.83 .0751      1

| 2            41    4.93   4.73    -.76     .20   -1.57   -.81   .51 -1.61 .1158      1

 

DIF Size = DIF Measure - baseline difficulty

 

The DIF Size (relative to the baseline item difficulty) for  Person group 1 on item 1 is .29 logits = -.47 - - .76

The DIF measure (relative to the baseline item difficulty) for Person group 2 on item 1 is -.81 logits =-1.57 - -.76

 

DIF Contrast = difference between item difficulties for the two groups =.29 - -.81 = 1.10 logits (same as above) = paired DIF effect for the two groups.

 

It is easiest to look at the DIF Size plot. The difference between the two lines is the DIF Contrast.

You can plot the difference from the Worksheet column D - column E.


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