What best distinguishes item difficulty from item discrimination in tests?

• A. Difficulty is the proportion of students who answer correctly; discrimination indicates how well an item differentiates high- from low-ability students.
• B. Difficulty and discrimination are the same concept and both measure item length.
• C. Difficulty measures student motivation; discrimination measures readability.
• D. Difficulty relates to test time; discrimination relates to scoring speed.

Answer: (A)

Two key properties of an item are its difficulty and its discrimination. Item difficulty is the proportion of students who answer correctly, so it tells you how easy or hard the item is for the whole group. Item discrimination measures how well the item separates higher- from lower-ability students—essentially whether those who perform well on the overall test are also the ones who get this item right.

For example, an item that many students get right is easy, but it may have low discrimination if both high- and low-ability students do well. Conversely, an item that a larger share of high-ability students gets right than low-ability students is good at discriminating between ability levels, even if it’s moderately difficult.

The other statements mix up what the measures reflect—length, motivation, readability, or timing—none of which describe how an item functions to distinguish between different levels of ability.