[Rasch] What is the relationship between number of items and the number of participants needed?

Matthew Enos matthewenos at mac.com
Thu Jun 20 03:45:28 EST 2013


Even though I am not actively engaged in measurement issues these days, I still glance at the Rasch exchanges -- and read the Mike L. posts with some care -- messages like this last one you sent capture my interest in science in general and measurement in particular. There is a wealth of thought in the exchanges on this line.

Discussions like the one below are especially interesting to me because they raise larger issues of measurement and science (and therefore of TRUTH). As a consummate educator, you always suggest, "...now, let's look into the subject a bit more deeply...." That's real teaching (and learning).

Marci and I miss the intellectual adventure in which you and Ben led us. At this point, we are not much involved, and have almost completely lost touch with Ben. Apparently, he is still in Chicago, but why is beyond me. The house is gone, but he continues in a Chicago hospital, right? Why is beyond me.

We hope all is well with you and Maureen. The news from Down Under is alternately beguiling and frightening. A wonderful continent, but with way too many things that bite or sting.

Take care of yourselves!


Matthew Enos
320 Crescent Drive
Glenview, IL 60025
Home: 847-983-0746

On Jun 18, 2013, at 6:13 PM, Mike Linacre wrote:

> Michael:
> You asked: "What is the relationship between number of items and the 
> number of participants needed?"
> Here are a few guidelines to launch our discussion:
> 1) at least 30 observations of each item and of each person, if possible.
> 2) at least 10 observations of each score-level on each rating scale.
> 3) since we are usually more concerned about item fit than person fit, 
> there must be more persons than items.
> Folks, please tell us about your own guidelines ....
> Mike L.
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> Rasch at acer.edu.au
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