In this exercise, all the items will have the same level of item discrimination but different levels of item difficulty. The intent is to relate the location of the item on the ability scale to its level of item difficulty.

(a) Use the function iccplot to plot an item characteristic curve of an item with very easy item difficulty and moderate item discrimination.

(b) From the graph it can be seen that the probability of correct response will be reasonably large over most of the ability scale. The item characteristic curve will be moderately steep in the lower part of the ability scale.

(c) After you have studied the curve, make sure to click the R console window to make it current and to type in a new command line.

(d) The next graph will be plotted on the same graph in the graphics window.

Type in:

> par(new=T)

(e) Now repeat steps a through d several times using items with moderate item discrimination and item difficulty levels of your choice (e.g., easy, medium, hard, very hard).

(f) The next graph will be plotted in a new graphics window.