Quartiles and percentiles are closely related concepts, which is exactly why so many IB Maths AI students mix them up. The confusion rarely comes from difficulty with calculation. Instead, it comes from unclear mental models and imprecise language.
Both quartiles and percentiles describe position within a data set, not frequency or value size. Quartiles split data into four equal parts, while percentiles split data into one hundred equal parts. Because they serve the same purpose at different resolutions, students often treat them as interchangeable, even though examiners do not.
One major source of confusion is memorisation without meaning. Students often remember that the median is both the second quartile and the 50th percentile, but they fail to understand why. Without that understanding, quartiles feel like “special percentiles” rather than a structured system, and explanations become inconsistent.
Language also causes problems. Students might say “the upper quartile means the top 25% of values,” which is incorrect. The upper quartile is a value, not a region. It is the value below which 75% of the data lies. This same mistake appears with percentiles, where students describe sections of data instead of positional cut-off points.
Another issue is overgeneralisation. Students sometimes assume that anything involving 25%, 50%, or 75% must be quartiles, and anything else must be percentiles. This shortcut fails in interpretation questions, especially when IB asks for comparisons or explanations rather than calculations.
Cumulative frequency graphs amplify this confusion. Quartiles and percentiles are read using the same method, so students focus on procedure and forget interpretation. When asked to explain results, they mix terminology, reversing meanings or using vague phrases like “most values.”
IB examiners are not strict about whether you use quartiles or percentiles — they are strict about whether you understand what they represent. Students who clearly state what proportion of data lies below a value consistently score higher, regardless of whether the value is a quartile or percentile.
Once students stop memorising labels and start focusing on positional meaning, the confusion fades quickly.
Frequently Asked Questions
Are quartiles just specific percentiles?
Yes. Quartiles correspond to the 25th, 50th, and 75th percentiles, but IB still expects correct terminology and explanation.
Can I use percentile language to explain quartiles?
Yes, as long as the explanation is precise and correct. Clarity matters more than labels.
What’s the most common mistake in exams?
Describing quartiles or percentiles as groups of data instead of positional values.
RevisionDojo Call to Action
Quartiles and percentiles only feel confusing when definitions replace understanding. RevisionDojo is the best platform for IB Maths AI because it trains students to explain statistical position clearly and consistently. If you want to stop losing marks to wording mistakes, RevisionDojo shows you how to think and explain like an examiner.
