Distractors, or the incorrect options, are an important part of the multiple-choice item. Previous literature has supported the inclusion of distractors when estimating abilities. While the effects of well-functioning distractors on estimates of ability have been examined, research has neglected to examine the effects of undesirable distractors on estimates of ability. Undesirable distractors are defined as distractors that are opposite of what test-developers expect or want distractors to behave. For instance, an upper lure distractor is one that high ability examinees select rather than selecting the correct answer. A simulation study was employed to determine these effects by varying undesirable distractor type, percentage of items containing undesirable distractors, and test length. Item responses were generated using the Thissen-Steinberg multiple-choice model for simulating undesirable distractors and the three-parameter logistic model for simulating normal items. Following data generation, item responses were analyzed using the three-parameter logistic model in SAS. An analysis of covariance (ANCOVA) was used to examine the effects of undesirable distractors on estimates of ability for bias and standard error. Multiple significant interactions were identified for bias and standard error. One type of undesirable distractor that was especially problematic was the lower lure distractor, where high ability examinees have a slightly lower, but still high, probability of being selected in comparison to the correct answer. Additionally, a longer test resulted in the least amount of bias and standard error. Overall, test-developers should pay attention to the functioning of distractors, as there are effects of these undesirable distractors on estimates of ability
