Abstract
A differential bundle function (DBF) is a situation in which examinees who are of the same ability but are from different groups are required to answer groups of items differently. DBF undermines the validity of the instrument if inadequately considered. The study examines the dimensionality of the 2017 NECO Mathematics items, determines the effect of DBF on 2017 Mathematics items concerning sex, and investigates the effect of DBF on 2017 Mathematics items concerning school ownership. This study explores Exploratory Structural Equation Modelling (ESEM), which permits the cross-loading of items that are not allowed in other models. The ex-post facto research design was adopted using secondary data, while six bundles were generated via the specification table. The population for the study comprised all 1,034,629 Senior School three students. A total of 815,104 students were selected using the simple random technique. The instrument for the study was 2017 NECO Mathematics paper three with a Cronbach's alpha of 0.82, and data were analysed using Mplus 7.4. The results revealed that the 2017 NECO Mathematics is multidimensional and items in the bundles possess construct validity as they functioned differentially to examinees' sex and school type. We recommend ESEM has a better approach to examining DBF on 2017 NECO Mathematics test items.