5. Data Collection and Correction 193
5 Data Collection & Correction Rasch modelling, like all forms of item response theory assessment, creates a common scale for a measurement framework. The values of the ability estimates of all persons can be related to each other and to the difficulty value of the items; the difficulty values of all items can be related to each other and also to the ability values for the persons. To do this, the model requires "connectedness" in the data. That is to say the data must either be a full data set (each item being applied to each learner) or it must be linked through what are called "anchor items" or "anchor persons." This study concerned the subjective assessment of learners by their teachers, and the analysis was to be undertaken with the many-faceted version of the Rasch rating scale model (Linacre 1989) which adds "judge" (rater) as a third facet to the conventional facets "item" (descriptor) and "person" Oearner). Therefore linking between judges (teachers) was also necessary. Ideally, this would be undertaken by having several teachers rate the same students in a complex matrix design, but this was hardly feasible with 100 teachers and 1,000 learners at the end of the academic year. Therefore, following Linacre's advice, a rating conference was used to achieve linking between the teachers themselves, and between the teachers and the questionnaires which they had not actually used and so to create a common measurement framework. The descriptors which had survived the pre-testing in the workshops were...
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