How many students got the question right?
Use actual responses from a single question to calculate its facility index. You only need two counts.
The formula and denominator
Facility index p = students correct ÷ students assessed on the question
Count students who received the question, including unanswered responses as not correct. Exclude absentees and students who were not assessed on it. If the question was dropped or has an unresolved key error, resolve that before interpreting the statistic.
Worked example: 18 out of 60
If 18 of 60 students answer correctly, p = 18 ÷ 60 = 0.30. That means 30% got it right. Higher values mean the question was easier for this particular group. There is no universal classroom cutoff for easy, moderate or hard.
What to do with a low value
Review the topic, question wording and answer key. A low value can reflect unfamiliar material or an ambiguous question. Compare it with your teaching goals rather than assuming it proves a poor question.
This is observed difficulty, not an estimate from the wording and not a discrimination index. The definition follows the University of Washington item-analysis guidance.
Review a full class test
Use ScanScore’s results to review questions alongside the answer key, or download the class result workbook to organise marks. The class result analysis guide explains the wider review workflow.
Common questions
Why does a higher index mean an easier question?
The facility index is the proportion who answered correctly. More correct responses produce a higher value, indicating that the question was easier for that group.
Should I include students who left the question blank?
Yes, if they received the question. Count their response as not correct. Exclude absentees and students who were not assessed on that question.
Does this measure discrimination?
No. Discrimination concerns how responses relate to overall performance. This tool calculates only the proportion correct for a single question.