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DFIT 8 for Windows

Nambury S. Raju, T. Chris Oshima & Allen H. Wolach

DFIT8 is a powerful Windows application designed to perform differential item functioning (DIF) analyses utilizing the DFIT framework that evaluates the differences between item response functions. This approach was originally suggested by Rudner, Getson, and Knight (1980), then further developed by Raju (1988) and Raju, van der Linden, & Fleer (1995). DFIT is an advanced approach based on item response theory (IRT) that compares the item response functions for groups of examinees to determine if a test item is performing significantly different for one group. DFIT8 includes a detailed manual with background on the DFIT method, instructions for installing and running the program, and descriptions of the input and output files, enabling you to quickly perform the analysis you need.

To learn more about the DFIT framework before purchasing the program, please read the NCME Instructional Module on DFIT in Educational Measurement: Issues and Practice, 27(3). More information is also available in the complete DFIT manual that ships with the software, which is available for free download.

Differential Functioning of Items and Tests

DFIT8 assesses differential functioning at both the item level (DIF) and the test level (DTF), and can be used for the dichotomous test data calibrated using 1-, 2-, or 3-parameter unidimensional IRT model as well as for polytomous test data (e.g., rating scale data) calibrated using Samejima’s graded response model. The DFIT approach also makes possible several applications that are very useful in DIF research (e.g., differential bundle functioning). The DFIT framework offers two types of DIF, non-compensatory DIF (NCDIF) and compensatory DIF (CDIF). Like most item-level DIF indices, NCDIF assumes that all items other than the studied item are DIF free, and can be used like any of the other DIF indices. CDIF on the other hand takes into account the pattern of DIF across items, and can be used to investigate the effect of removing the DIF items on DTF.

DFIT8’s Windows Interface

In contrast to earlier versions of DFIT, DFIT8 uses a complete point-and-click Windows interface. The following example shows the main options window for a set of dichotomously scored items:

Main options window for a set of dichotomously scored items

For polytomously scored items using the graded response model, the following options are available:

Options window for a set of polytomously scored items

DFIT output is provided in well structured simple tabular formats:

Output window.

Significance Testing in DFIT

DFIT8 employs a recently developed significance test for NCDIF and DTF. The test is called the item parameter replication (IPR) method and provides a means of deriving cutoff values that are tailored to a particular data set. The IPR method begins with estimates of item parameters for the focal group and the sampling variances and covariances of these estimates. Based on these initial estimates, a large number of replications (typically 1,000 pairs) of item parameters are simulated. Then the cutoff value for each alpha level is determined by the empirical sampling distribution of NCDIF/DTF obtained under the null hypothesis that focal and reference groups have identical parameters. The IPR method produces cutoff values for each item.

The significance of CDIF is not tested directly. Instead, items with large CDIF are removed one by one until DTF reaches non-significance by a chi-square test. Those removed CDIF items are then considered significant.

Output Significance.

The IPR is easy to use for practitioners because it is implemented within the DFIT software program; practitioners are only required to provide estimates of item parameters, their variances and covariances, and ability estimates for the focal or reference group.

System Requirements

Microsoft Windows 2000/XP/Vista
7 Mb free disk space
2 Mb free RAM

More Information

The complete DFIT8 PDF manual that ships with the software is available for download. This manual provides extensive coverage of both the background and operatinon of DFIT8.

10-Day Trial copy

A fully functioning trial copy of DFIT8 is available for download. This package includes the complete PDF manual. The trial copy can be unlocked for permanent use when payment arrangements have been completed.

 

*The student lab license is valid for 120 days, and requires the purchase of at least one regular academic license for the course instructor.

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