6. Constructing the Scale 223
6 Constructing the Scale As indicated earlier, the analysis has been undertaken with the many-faceted Rasch Rating Scale Model (Linacre 1989). In a simple Rasch analysis of di- chotomous (right/wrong) test items, there are two facets: "person" and "item." \Vhen the severity of the rater is taken into account in the cali- bration, as is possible with the FACETS program, "judge" becomes a third facet. It is also possible to design other facets in the measurement situation such as "occasion" in order to, for example, investigate the stability of the severity of examination judges of at different administration sessions in the same examination (Lunz and Stahl 1990; Stahl, Lunz and Wright 1991; Stahl and Lunz 1992). In this case the two occasions are (1) the rating by each teacher of 10 learners from their own classes and (2) the rating by each teacher of video performances of 23 of the learners in the survey at the rat- ing conference. It is also possible to include demographic facets in the data so that once the measurement framework has been established, and values attributed to the measurement facets (here: Teacher, Occasion, Learner, Item), the ef- fects of demographic information like age, sex, race (here: Educational Sector, Language Region) can be calculated. As well as overall measures for the demographic facets defined, it is also possible to run Bias studies in or- der to identify bias against specific individuals or bias by certain items (here descriptors) for or against specific groups, which would...
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