Difficulty plots: DIF analysis without the partial credit model
From summer 2017, the preferred method for vertical scaling of tiered exams will be chained equipercentile equating. AQA has previously used the partial credit model (PCM) to perform vertical scaling. For either method, differential item functioning (DIF) should be assessed among the common items, ideally at the time of the award.
Any items that perform very differently on the foundation and the higher tier might need to be dropped from the common item pool to ensure a fair link between the two tiers. DIF is fairly straightforward to assess when using the PCM; this report illustrates a simple method, using difficulty plots, to assess DIF without fitting the PCM. The two approaches are compared over 12 tiered GCSE papers from the summer 2016 series.