[Rasch] Interpreting item maps of polytomously scored items

Snider-Lotz, Tom tsnider-lotz at previsor.com
Thu Aug 24 04:36:25 EST 2006

Hello everybody --

My question is, what is the best statement to explain the relationship
between a person and an polytomously-scored item when they are at the
same spot on the logit scale?

I believe that, when items are scored dichotomously, it is appropriate
to say a person with an ability level of X logits has a 50% chance of
correctly answering an item with a difficulty of X logits.  What is the
corresponding statement when the item is scored polytomously?

Concretely, I've got an Emotional Maturity scale with items that are
scored 1 to 5 on a Likert scale.  A higher score means greater emotional
maturity.  Winsteps table 1.2 generates an item map with a vertical
logit scale, with persons graphed on the left and items graphed on the
right.  Each item shows once on the display (i.e., it does not show the
different thresholds).  At 0.8 logits there are several people on the
left and item 8 across from them on the right.  Given this, what does
this say about those persons' likely response to item 8?  Are they most
likely to choose the middle choice on the scale (3 out of 5)?  Most
likely to be at a level equal to the mean response to item 8? Or what .
. . ?

Thanks, everybody.

 -- Tom Snider-Lotz


Thomas G. Snider-Lotz, Ph.D.
Principal Scientist
1805 Old Alabama Road, Suite 150
Roswell, GA 30076
T:678.832.0555 F:770.642.6115
tsnider-lotz at previsor.com
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