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

Abstract

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