﻿ Non-uniform DIF tables

# Non-uniform DIF tables

When items exhibit different difficulties for different person groups this is called "item bias" or "differential item functioning", DIF. When the DIF is the same for all ability levels of the two groups this is termed Uniform DIF. Table 30 reports this. When DIF differs with ability levels, this is termed Non-Uniform DIF, NUDIF. This is discussed here.

Non-uniform DIF is difficult to interpret. Please start by looking at

This will show you which items have an interaction between item difficulty, group membership and ability.

If you see an item for which the item difficulty is noticeably different for high and low performers in a group, then Table 30.1 will report the statistical significance of the difference.

DIF class specification is: DIF=\$S1W1+MA2

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

| Examinee  Obs-Exp   DIF   DIF   Examinee  Obs-Exp   DIF   DIF      DIF    JOINT     Welch      Mantel-Haenszel Size Item         |

| CLASS     Average MEASURE S.E.  CLASS     Average MEASURE S.E.  CONTRAST  S.E.   t  d.f. Prob. Chi-squ Prob. CUMLOR Number  Name |

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

| 21           .00  -.1950 .1151  22          -.13   .5092 .1836   -.7042 .2166 -3.25 300 .0013   .0000 1.000             3 1     |

Conceptually, all DIF is non-uniform. "Uniform DIF" = "Average non-uniform DIF". For instance, in the Mantel-Haenszel DIF computation, every raw-score stratification reports a different DIF estimate. The Mantel-Haenszel  statistic averages these estimates. Winsteps reports uniform DIF as a summary of a non-uniform DIF distribution in the same way that the mean is a summary of a sample distribution.

1) Look at the empirical ICC (Graphs menu, Non-uniform DIF). Is the empirical ICC clearly flatter or steeper than the model ICC for a classification group? Suppose that is group F

2) In the Winsteps DIF table 30.1, we expect F1 vs. F2 to be both a large DIF contrast (>1.0 logits) and to be statistically significant (p≤.05).

3) There should be a substantive reason for deciding that the NUDIF is not merely a statistical accident.

Example, the division sign, ÷, is very difficult for those students who have not learned its meaning, and easy for those who have. So it exhibits NUDIF across grade-levels.

In detail:

1. Simple NUDIF reporting. In Table 30, specify both a person class column with DIF= and the number of ability strata to be reported, e.g., \$MA2, which reports 2 levels of ability, the top and bottom half of the ability range. Here is the dialog box:

Here is the plot:

The blue box flags an example of non-uniform DIF. It is"F2" where "F" (@GENDER) is the females, and "2" (MA2) the upper ability group. "F1" is "F' ability group "1", the lower ability group. The non-uniform DIF numbers are shown in Table 30.2:

Some person classes have extreme scores. These are shown in the green boxes, and are indicated by "<" for minimum extreme scores and ">" for maximum extreme scores. The plotted DIF measures are shown in the blue box, with the DIF size relative to the overall item difficulty level in the adjacent column.

Table 30.1 reports if there is pairwise DIF between CLASSES/GROUPS.

2. For more insights, plot the non-uniform DIF ICCs.

Help for Winsteps Rasch Measurement Software: www.winsteps.com. Author: John Michael Linacre

 Forum Rasch Measurement Forum to discuss any Rasch-related topic

Rasch Publications
Rasch Measurement Transactions (free, online) Rasch Measurement research papers (free, online) Probabilistic Models for Some Intelligence and Attainment Tests, Georg Rasch Applying the Rasch Model 3rd. Ed., Bond & Fox Best Test Design, Wright & Stone
Rating Scale Analysis, Wright & Masters Introduction to Rasch Measurement, E. Smith & R. Smith Introduction to Many-Facet Rasch Measurement, Thomas Eckes Invariant Measurement with Raters and Rating Scales: Rasch Models for Rater-Mediated Assessments, George Engelhard, Jr. & Stefanie Wind Statistical Analyses for Language Testers, Rita Green
Rasch Models: Foundations, Recent Developments, and Applications, Fischer & Molenaar Journal of Applied Measurement Rasch models for measurement, David Andrich Constructing Measures, Mark Wilson Rasch Analysis in the Human Sciences, Boone, Stave, Yale
in Spanish: Análisis de Rasch para todos, Agustín Tristán Mediciones, Posicionamientos y Diagnósticos Competitivos, Juan Ramón Oreja Rodríguez
Winsteps Tutorials Facets Tutorials Rasch Discussion Groups

Coming Rasch-related Events
Jan. 5 - Feb. 2, 2018, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Core Topics (E. Smith, Winsteps), www.statistics.com
Jan. 10-16, 2018, Wed.-Tues. In-person workshop: Advanced Course in Rasch Measurement Theory and the application of RUMM2030, Perth, Australia (D. Andrich), Announcement
Jan. 17-19, 2018, Wed.-Fri. Rasch Conference: Seventh International Conference on Probabilistic Models for Measurement, Matilda Bay Club, Perth, Australia, Website
Jan. 22-24, 2018, Mon-Wed. In-person workshop: Rasch Measurement for Everybody en español (A. Tristan, Winsteps), San Luis Potosi, Mexico. www.ieia.com.mx
April 10-12, 2018, Tues.-Thurs. Rasch Conference: IOMW, New York, NY, www.iomw.org
April 13-17, 2018, Fri.-Tues. AERA, New York, NY, www.aera.net
May 22 - 24, 2018, Tues.-Thur. EALTA 2018 pre-conference workshop (Introduction to Rasch measurement using WINSTEPS and FACETS, Thomas Eckes & Frank Weiss-Motz), https://ealta2018.testdaf.de
May 25 - June 22, 2018, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Core Topics (E. Smith, Winsteps), www.statistics.com
June 27 - 29, 2018, Wed.-Fri. Measurement at the Crossroads: History, philosophy and sociology of measurement, Paris, France., https://measurement2018.sciencesconf.org
June 29 - July 27, 2018, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Further Topics (E. Smith, Winsteps), www.statistics.com
July 25 - July 27, 2018, Wed.-Fri. Pacific-Rim Objective Measurement Symposium (PROMS), (Preconference workshops July 23-24, 2018) Fudan University, Shanghai, China "Applying Rasch Measurement in Language Assessment and across the Human Sciences" www.promsociety.org
Aug. 10 - Sept. 7, 2018, Fri.-Fri. On-line workshop: Many-Facet Rasch Measurement (E. Smith, Facets), www.statistics.com
Sept. 3 - 6, 2018, Mon.-Thurs. IMEKO World Congress, Belfast, Northern Ireland www.imeko2018.org
Oct. 12 - Nov. 9, 2018, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Core Topics (E. Smith, Winsteps), www.statistics.com

Our current URL is www.winsteps.com