[Rasch] Rasch S.E at the cut score...reciprocal?

Mike Linacre mike at winsteps.com
Fri Nov 1 09:37:41 EST 2013


Imogene:

Your logic about the S.E. of raw scores is correct: smallest at the 
extremes, biggest in the center.

See http://www.rasch.org/rmt/rmt204f.htm

To have an S.E. as small as 0.05 logits, the test must have at least 
1/(0.05*0.05*0.25) = 1600 dichotomous items, so that 40 is 2.5% of the 
raw score range.

Mike L.

On 11/1/2013 8:23 AM, Imogene Rothnie wrote:
>
> I have recently read some Rasch digests about error at the cut point 
> of the test. These suggest that to convert the Rasch logit S.E. at the 
> cut point of the test to raw score units (or approximately)  one 
> should take the reciprocal of the S.E. of the nominated logit value.
>
> HOWEVER, as the S.E. gets smaller (good) the reciprocal gets bigger! 
> And so the raw score range within the boderzone becomes larger. I must 
> be missing something?
>
> E.g we have a test with very small S.E. at cut logit measure of 
> 0.05...  1/0.05 is 20- a borderzone of 40marks...whereas an S.E. of 
> 0.15, hence 1/0.15  would give you a raw score range of 14...
>
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