A method and system for item exposure control in computer based testing. A set of exposure control parameters representing the probability that an item selected from an item pool is administered to a test taker for each item in the pool is generated through computer based test simulation. The exposure control parameters can be initialized using values less than one that reflect the expected value after the test simulations are complete. The items in the item pool are ordered from most desirable to least desirable to administer based on predefined criteria. The set of exposure control parameters are used to generate operant probabilities associated with each of the items and represent the probability that the item is selected to be administered given that no more desirable item has been administered first. Items are randomly selected to be administered based on their corresponding operant probabilities. Stimuli may be included in the item list, and when randomly selected, the set of items associated with the stimulus are administered. The operant probabilities can be adjusted by item security parameters which are defined based on a desired measure of test security. A system for carrying out the steps includes an exposure control parameter generator, an item selector, and an item administrator.