Using recursive partitioning Rasch trees to investigate differential item functioning in second language reading tests

The present study applied recursive partitioning Rasch trees to a large-scale reading comprehension test (n = 1550) to identify sources of DIF. Rasch trees divide the sample by subjecting the data to recursive non-linear partitioning and estimate item difficulty per partition. The variables used in the recursive partitioning of the data were vocabulary and grammar knowledge and gender of the test takers. This generated 11 non-prespecified DIF groups, for which the item difficulty parameters varied significantly. This is grounded within the third generation of DIF analysis and it is argued that DIF induced by the readers' vocabulary and grammar knowledge is not construct-irrelevant. In addition, only 204 (13.16%) test takers who had significantly high grammar scores were affected by gender DIF. This suggests that DIF caused by manifest variables only influences certain subgroups of test takers with specific ability profiles, thus creating a complex network of relationships between construct-relevant and -irrelevant variables.