Subtest scoring and weighting
A test or protocol may consist of a series of subtests. You want each person to have a separate measure on each subtest, but in the same frame of reference as the overall test measures.
First, code each item in its item label with what subtest it belongs to.
A. Using DPF.
1. Analyze all the data together.
2. Reporting of subtests.
Reporting subtest person measures: a measure for each person on each subtest can be obtained by specifying the subtest identification column in the item label in DPF= and producing Table 31 or the DPF plot.
B. Using separate analyses.
1. Analyze all the data together. Produce an IFILE=if.txt. Produce a PFILE= for global measures.
2. Anchor all items IAFILE=if.txt.
Select one subtest: ISELECT=subtest code
Produce IFILE= and PFILE for the subtest.
3. Repeat for the other subtests.
The different subsections have different maximum scores, but you want each subsection to be equally influential in estimating the final measures.
1) Do a standard Rasch analysis to investigate coding problems, misfit, reliability, etc.
2) Weight the items in the different sections so that the overall weights for each section are the same.
Let's suppose that the items are all dichotomies scored 0,1, then
1-40 1 ; the 40 items in section 1 are given a weight of 1, so weight of section 1 = 40*1 = 40
41-70 1.333 ; the 30 items in section 2 are given a weight of 1.333, so weight of section 2 = 30*1.333 = 39.99
71-120 .8 ; the 50 items in section 3 are given a weight of 0.8, so weight of section 3 = 50*0.8 = 40
We want to maintain close to the original overall weight, so that the final reliabilities and standard error have reasonable values.
3) Perform the weighted analysis to report the final person measures.
There are other effective approaches. For instance, perform a separate Rasch analysis for each section, and then combine the subtest measures using www.rasch.org/rmt/rmt83f.htm:
combined measure = sum(section measure/S.E.(section measure))/sum(1/S.E.(section measure))
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|Rasch Measurement Transactions (free, online)||Rasch Measurement research papers (free, online)||Probabilistic Models for Some Intelligence and Attainment Tests, Georg Rasch||Applying the Rasch Model 3rd. Ed., Bond & Fox||Best Test Design, Wright & Stone|
|Rating Scale Analysis, Wright & Masters||Introduction to Rasch Measurement, E. Smith & R. Smith||Introduction to Many-Facet Rasch Measurement, Thomas Eckes||Invariant Measurement with Raters and Rating Scales: Rasch Models for Rater-Mediated Assessments, George Engelhard, Jr. & Stefanie Wind||Statistical Analyses for Language Testers, Rita Green|
|Rasch Models: Foundations, Recent Developments, and Applications, Fischer & Molenaar||Journal of Applied Measurement||Rasch models for measurement, David Andrich||Constructing Measures, Mark Wilson||Rasch Analysis in the Human Sciences, Boone, Stave, Yale|
|in Spanish:||Análisis de Rasch para todos, Agustín Tristán||Mediciones, Posicionamientos y Diagnósticos Competitivos, Juan Ramón Oreja Rodríguez|
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