The discrimination index of an item is a crucial metric in item analysis that shows how well the item differentiates high and low achievers. It measures the extent to which an item separates students who know the material from those who do not. Typically, this is calculated by comparing the performance of students in the upper-scoring group (e.g., the top 27% on the overall test) with those in the lower-scoring group (e.g., the bottom 27%).
A high positive discrimination index indicates that students who performed well on the overall test were significantly more likely to answer that specific item correctly than students who performed poorly. This suggests the item is effective and contributes positively to the test's ability to measure the intended construct. Conversely, a low or negative discrimination index signals a problematic item that may be confusing, flawed, or not measuring what it should. Effective discrimination is vital for creating valid and reliable assessments that accurately distinguish between different levels of student proficiency.