Overcoming vertical equating complications in the calibration of an integer ability scale for measuring outcomes of a...

The measurement complexities emerging from vertical equating in an educational experiment aiming at an advance in the curriculum are addressed, when calibrating an 'integer ability' scale for year 5 students from Greater Manchester based both on primary (years 5 and 6) and high school (years 7 and 8) data. The need for such a calibration resulted from experimental teaching of 'high school content' in primary school. Substantial Rasch differential item functioning (DIF) arose in the vertical equating between primary and high school in our initial 'all-on-all' 'concurrent' calibration. A second 'Primary anchored-and-extended' calibration which substantially overcame DIF problems is shown to be preferable for our teaching experiment. The relevant methodological challenges and the techniques adopted are discussed. The solution provided might be useful to researchers for educational experiments targeting an advance in the curriculum.