An Assessment of the Effects of Item Difficulty and Examinee Ability on the Effectiveness of LZ Appropriateness Index

Abstract

This study investigated the effectiveness of LZ appropriateness index in detecting aberrant response patterns under nine combinations of item difficulty and examinee’s ability distributions, type of aberrance, and level of aberrance. Data was generated in nine combinations of item difficulty and examinee ability to simulate the responses of 2000 non-aberrant examinees’ response patterns to a 60-item test according to three-parameter model. Three uniform distributions of item difficulty were used. Two samples each consisting of 500 normal response vectors (one for spuriously low and one for spuriously high modifications) were also generated in each of the nine combinations and subjected to spurious treatment. An examinee with a spuriously high test score was simulated by selecting 20% or 10% of the examinee's original responses without replacement and changing incorrect answers to correct, but they were left unchanged if correct. An examinee with a spuriously low test score was simulated by first randomly selecting 20% or 10% of the examinee's original responses without replacement and changing correct responses to incorrect, but they were left unchanged if incorrect. LZ appropriateness index was then computed for the aberrant response vectors. The effectiveness of LZ index was evaluated by examining the extent to which it separated normal and aberrant response vectors solely on the basis of appropriateness index scores. The percentile estimates obtained for each index at each false positive rate were used as cutoff scores. The LZ index identified higher proportions of aberrant response patterns in the 20% spuriously low treatment samples than in the 20% spuriously high treatment samples. Ten percent spuriously low aberrant response samples were also found to be more detectable than the 10% spuriously high aberrant response patterns. The detection rates of the 20% and the 10% spuriously high aberrant response patterns by LZ index were found to be higher under high item difficulty parameters, and were found to be low under the low item difficulty parameters. This is not surprising as it is expected that more responses are changed from incorrect to correct and fewer responses are changed from correct to incorrect under high item difficulty parameters. The 20% and the 10% spuriously low aberrant response patterns were also more detectable under the low item difficulty parameters because more responses are changed from correct to incorrect and fewer are changed from incorrect to correct under the low item difficulty parameters.