Wednesday, September 19, 2012

Rasch Model Convergence Logit Black Box

This view of Fall8850a.txt Winsteps results has been visited before. The difference is that now I have an idea of what the charts are showing in three ways:

1. The plot of student abilities for the rating scale analysis passes directly through the zero logit location indicating a good convergence. 2. The plots for student abilities and item difficulties are perfectly straight lines (considering my rounding errors), which again shows a good convergence. 3. The two lines are parallel, another indicator of a good convergence.

Culling increased the distribution spread to higher values for both analyses, as shown in the previous post. The plot of student abilities for culled partial credit did not pass through the zero logit location. Removing 7 students and 7 items has resulted in a poor convergence.

The item difficulty plots for the partial credit analysis are very different from those from the rating scale analysis. Here items starting convergence with the same difficulty can end up at various ending locations. The lowest and the highest locations are plotted for each item.

This post must end, as a comparison of Rasch IRT and PUP CTT results cannot be made directly between logit and normal scale values. 

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