WHAT IS A WELL-FUNCTIONING TEST ITEM?

• how many students got it correct? (difficulty)
• which students got it correct? (discrimination)

Difficulty level = # of students choosing correctly = p
                                        total # of students

Items should have some power of DISCRIMINATION. A good item will be answered correctly more often by students who scored well on the exam overall and less often by students who scored poorly. Therefore the item functions to discriminate those who knew the content and those who found the answer in some other way.

DISCRIMINATION = high 1/2 overall scores who answered item correctly (minus)
INDEX                            low 1/2 overall scores who answered item correctly

Example:
Item X DI = (12 of 15) - (7 of 15) = .33 (e.g., a good item)
                         (30/2) = 15