In this exercise, all the items will have the same level of item difficulty but different levels of item discrimination. The intent is to relate the steepness of the curve to the level of item discrimination.

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

(b) From the graph it can be seen that the probability of correct response will be small at low ability levels and large at high ability levels. The item characteristic curve will be moderately steep in the middle 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 medium item difficulty for each item and item discrimination levels of your choice (e.g., none, low, high, perfect).

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