7. Interpreting the Scale 271
7 Interpreting the Scale 212 descriptors had now been calibrated to estimated difficulties on a common logit scale running from -5.68 to 4.68. Having successfully con- structed a scale of items, the next step was to investigate it firstly in order to check that it indeed made sense, that similar content was calibrated in a co- herent fashion, and secondly in order to present it in a form in which it could be meaningful to other users. In effect this meant dividing the scale up into a number of bands or levels, which in turn involved setting cut-off points, and then seeing (a) whether those levels had coherent content (b) whether progress up the scale in each category was logical. Setting Cut-offs between Levels The number of levels or strata which can be identified in a set of data is connected to the question of reliability. Pollitt explains a calculation with Table 7.1: Reliability and the Number of Strata in Data r Pollitt 1991 Fisher 1992 Bands Distinct Strata 0.98 9 0.97 8 0.96 10.1 7 0.94 5 0.90 6.3 4 0.80 4.3 3 0.70 2 0.50 1 which one can derive the decision capability from any test (or rating) from its reliability coefficient (Pollitt 1991: 90). Fisher (1992) offers similar in- formation and his table and Pollitt's compare as in Table 7.1. 272 The Development of a Common Framework S.-ale oJLmguage Profldenry The reliability statistic for the full integrated analysis (simulated Cronbach Alpha) was 0.97, which, according to Pollitt...
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