Correction of Differentially Functioning Items: Basis for Maintaining and Enhancing Test Validity and Reliability

Jose Q. Pedrajita


This study looked into differentially functioning items in a Chemistry Achievement Test. It also examined the effect of eliminating differentially functioning items on the content and concurrent validity, and internal consistency reliability of the test. Test scores of two hundred junior high school students matched on school type were subjected to Differential Item Functioning (DIF) analysis. One hundred students came from a public school, while the other 100 were private school examinees. The descriptive-comparative research design utilizing differential item functioning analysis and validity and reliability analysis was employed. The Chi-Square, Distractor Response Analysis, Logistic Regression, and the Mantel-Haenszel Statistic were the methods used in the DIF analysis. A six-point scale ranging from inadequate to adequate was used to assess the content validity of the test. Pearson r was used in the concurrent validity analysis. The KR-20 formula was used for estimating the internal consistency reliability of the test. The findings revealed the presence of differentially functioning items between the public and private school examinees. The DIF methods differed in the number of differentially functioning items identified. However, there was a high degree of correspondence between the Logistic Regression and Mantel-Haenszel Statistic. After the elimination of the differentially functioning items, the content and the concurrent validity, and the internal consistency reliability differed per DIF method used. The content validity of the test differed ranging from slightly adequate to moderately adequate in the number of items retained. The concurrent validity of the test also differed but all were positive and indicate moderate relationship between the examinees’ test scores and their GPA in Science III. Likewise, the internal consistency reliability of the test differed. The more differentially functioning items eliminated, the lesser was the content and concurrent validity, and internal consistency reliability of the test becomes. Elimination of differentially functioning items diminishes content and concurrent validity, and internal consistency reliability, but could be use as basis in enhancing content, concurrent as well as internal consistency reliability by replacing eliminated DIF items.

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