Determining differential item functioning and its effect on the test scores of selected PIB indexes, using item response theory techniques

Abstract

The objective of this article is to present the results of an investigation into the item and test characteristics of two tests of the Potential Index Batteries (PIB) in terms of differential item functioning (DIF) and the effect thereof on test scores of different race groups. The English Vocabulary (Index 12) and Spelling Tests (Index 22) of the PIB were analysed for white, black and coloured South Africans. Item response theory (IRT) methods were used to identify items which function differentially for white, black and coloured race groups. The effects of the differences between the item characteristic curves (ICCs) of the three race groups on the test characteristic curve (TCCs) were studied. The items identified as biased (DIF) appeared to have a negligible effect on the test scores of Index 12 and Index 22 at the different ability levels for the groups considered. It can be concluded that the tests do not appear to discriminate unfairly, due to DIF, against race groups. OPSOMMING: Die doel van hierdie artikel is om die resultate van ‘n ondersoek na die item- en toetseienskappe van twee PIB (Potential Index Batteries) toetse in terme van itemsydigheid en die invloed wat dit op die toetstellings van rassegroepe het, weer te gee. Die Potential Index Batteries (PIB) se Engelse Woordeskat (Index 12) en Spellingtoetse (Index 22) is ten opsigte van blanke, swart en gekleurde Suid- Afrikaners ontleed. Itemresponsteorie (IRT) is gebruik om items te identifiseer wat as sydig (DIF) vir die onderskeie rassegroepe beskou kan word. Die effek van die verskille in itemkarakteristieke kurwes van drie groepe is op die toetskarakteristieke kurwe ondersoek. Dit blyk dat die items wat as DIF geïdentifiseer is, ‘n onbenullige effek op die toetstellings van Index 12 en 22 vir die verskillende vermoënsvlakke van die drie groepe het. Dit kom voor dat die toetse nie as gevolg van DIF onbillik teen blank, swart of kleurlingrassegroepe diskrimineer nie.