A negative point-biserial means the students who did best on the exam overall were more likely to miss this item than the students who did worst. The item is pulling against everything else on the test. Content that only weak students know does not really exist, so a negative value is almost always a signal that something in the item — not the students — is off.
The three usual causes
1. The key is wrong. The most common cause and the easiest to check. Look at the option-level point-biserials in your export: if one distractor has a clearly positive value while the keyed answer is negative, your best students chose that distractor. Read the item again as if you had never seen the key.
2. Two options are defensible. Strong students see a nuance you didn't intend. A pain-assessment item that keys "use a validated behavioral scale" while also offering "ask a family member familiar with the patient" will split your top students, because both are things a good nurse does. The option-level numbers show this as two options with positive point-biserials and no clear winner.
3. The stem reads two ways. Negatively-worded stems ("Which is not an example of…"), "best" versus "first", and stems that bury the actual question at the end all reward students who read fast and punish students who read carefully. If the item is also very easy (P-value above 0.85) and still negative, this is the likely cause — nearly everyone got it, but the handful who missed it were your top students.
How to confirm which one it is
Open the distractor table for the item and look at four things:
- Which option has the highest point-biserial? If it isn't the key, start with cause 1.
- Are two options both positive? Cause 2.
- Is the P-value very high and the point-biserial negative? Cause 3.
- Were any distractors chosen by nobody? The item is effectively a two- or three-option item, which makes any ambiguity between the remaining options hit harder.
What to do with it
- For the exam already given: rescore if the key was wrong; accept both options if two were defensible; leave it if the stem was merely awkward and the key is right.
- For the item bank: rewrite the stem as a direct question, remove absolute and negative wording, replace dead distractors, and have a colleague answer the item without the key before it goes back in.
- For the exam as a whole: a single negative item in a 50-item exam is normal. Five or six is a pattern — usually a batch of items written or edited at the same time, and worth reviewing together.
Where MapMyExam fits
The Exam Health Report flags every negative and weak item, shows the option-level evidence beside it, and gives you a plain-language diagnosis and a rewrite recommendation for each one — reviewed by a nurse educator before it's sent. See pricing.