Item difficulty/percentile ranking and z-score

eija

New Member
#1
Hi

I am taking a course in psychometrics and am studying for my final exam, which is in two days.
I was going over some old exam questions and found one that confuses me:

"What is the z-score of an item, which 16% answered correctly"

Instinctively, I want to say 1.0.

I am then thinking that the items are percentile ranked (according to difficulty, which according to CTT would be 0.16) and this particular item's rank would be 84, which corresponds to z-score 1.0.

But I am confused... is it even possible to rank items like this, according to difficulty? Might this be a trick question?
I have turned and twisted this all day, looked at it from every perspective I could think of, including ITR, but this is what makes the most sense.
I just cannot find any information on whether or not this is a valid way to go about things.

If someone has anything to say about this, I would much appreciate it!

Eija