[Rasch] Rasch S.E at the cut score...reciprocal?
mike at winsteps.com
Fri Nov 1 09:37:41 EST 2013
Your logic about the S.E. of raw scores is correct: smallest at the
extremes, biggest in the center.
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.
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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