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The item difficulty index measures how easy a question is by determining the proportion of students who got it right. The item discrimination index measures how well a test question can help examiners differentiate between test takers who have attained mastery of the material and those who have not.
The item difficulty index ranges from 0 to 100; the higher the value, the easier the question. When an alternative is worth other than a single point, or when there is more than one correct alternative per question, the item difficulty is the average score on that item divided by the highest number of points for any one alternative.
Items that are below .25 are the most difficult, and items above .75 cross the probability threshold for guessing in items with 4 answer choices. Why?
Difficulty Index - Teachers produce a difficulty index for a test item by calculating the proportion of students in class who got an item correct. (The name of this index is counter-intuitive, as one actually gets a measure of how easy the item is, not the difficulty of the item.)
Item analysis typically focuses on four major pieces of information: test score reliability, item difficulty, item discrimination, and distractor information. No single piece should be examined independent of the others.
To determine the difficulty level of test items, a measure called the Difficulty Index is used. This measure asks teachers to calculate the proportion of students who answered the test item accurately.
An item analysis allows one to determine whether a multiple-choice question discriminates between students who know the material from those who do not and consists of calculating two indices for each question: a difficulty index and a discrimination index (Salkind, 2017).
Difficulty Index: The difficulty index is a number between 0 and 1 that indicates how hard the question is. It is the proportion of students who answered correctly.
In classical test theory, a common item statistic is the item’s difficulty index, or “p value.” Given many psychometricians’ notoriously poor spelling, might this be due to thinking that “difficulty” starts with p? Actually, the p stands for the proportion of participants who got the item correct.
for example, if you have an item difficulty of 0.95, it doesn't mean it's extremely difficult. It means it's extremely easy because that 0.95 represents the fact that 95% of the students got it correct. Now let's talk about item Discrimination Index, or DI. When we subtract the proportion of