Inestimable elements or excessive logit ranges

Facets reports elements are inestimable when the data cannot produce an estimate.

 

1. Inestimable due to all observations being in the same category of a partial-credit item.

Solution: Use SAFILE= to anchor the thresholds of the item at reasonable values.

 

2. A frequently-encountered problem in the analysis of paired-comparison data is an almost Guttman ordering of the pairings. This can lead to unrealistically huge logit ranges for the estimates of the elements or inestimable elements.

 

To solve this problem, we apply a little Bayesian logic. We know that the range of paired performances is not exceedingly wide, and we can also easily imagine a performance better than any of those being paired, and also a performance worse than any of the those being paired. Let's hypothesize that a reasonable logit distance between those two hypothetical performances is, say, 20 logits.

 

http://www.rasch.org/rmt/rmt151w.htm is a parallel situation for sports teams.

 

1.Hypothesize a "best"performance against which every other  performance is worse. Anchor it at 10 logits.

2.Hypothesize a "worst" performance against which every other performance is better. Anchor it at -10 logits

3.Hypothesize a dummy judge who compares the best and worst performances against all the other performances.

4.Include these dummy observations in the analysis.
 

5.Analyze the actual observations + the dummy observations. The analysis should make sense, and the logit range of the performances will be about 20 logits. For reporting, we don't want the dummy material, so we write an Anchorfile= from this analysis.
 

6.We then use the Anchorfile as the Facets specification file, commenting out the "best" and "worst" performance elements and the dummy judge. We analyze only the actual observations. All the elements are anchored at their estimates from the actual+dummy analysis. In this anchored analysis, the "displacements" indicate the impact of the dummy data on the estimates.
 

7.If you perceive that the logit range of 20 logits is too big or too small, please adjust the "best" and "worst" anchored values.


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