Mixing up “and” versus “or” is one of the most common — and costly — mistakes in IB Maths AI probability. What makes this error frustrating is that students often understand both ideas separately, yet still choose the wrong one under exam pressure.
The main reason this happens is language overload. In everyday speech, “and” and “or” are flexible and sometimes interchangeable. In probability, they are not. “And” refers to events occurring together, while “or” refers to at least one of multiple events occurring. IB relies on this precision, but students often read questions too quickly to notice the difference.
Another issue is that “or” in probability is usually inclusive, not exclusive. When students hear “or,” they instinctively think “one or the other, but not both.” In IB probability, “or” almost always means one event, the other, or both. Forgetting this leads to missing overlap and underestimating probabilities.
Students also struggle because “and” questions often involve conditional thinking, even when it is not stated explicitly. For example, finding the probability of A and B requires understanding how events interact. If students default to adding probabilities instead of multiplying or conditioning, it reveals a surface-level reading of the question.
Diagrams again play a critical role. Venn diagrams make “and” visible as overlap, while “or” includes multiple regions. Tree diagrams clarify sequences where “and” refers to consecutive outcomes. Students who skip diagrams are far more likely to confuse operations.
Time pressure magnifies the problem. When students rush, they latch onto keywords like “probability” and “event” and stop processing the logical structure of the sentence. IB exploits this by writing questions that look simple but require careful reading.
IB examiners are not trying to trick students — they are testing whether students can translate language into mathematical meaning. A student who pauses, rewrites the situation in their own words, and chooses the operation deliberately will almost always outperform a faster student who relies on instinct.
Once students learn to slow down and treat “and” and “or” as structural signals rather than filler words, this entire category of mistakes largely disappears.
Frequently Asked Questions
Is “or” always inclusive in IB probability?
Yes, unless the question explicitly states otherwise. Always assume overlap is included.
What’s the fastest way to avoid this mistake?
Rewrite the event in words and sketch a quick diagram before calculating.
Do examiners award method marks if the operation is wrong?
Usually not. Choosing “and” versus “or” correctly is foundational.
RevisionDojo Call to Action
Probability errors often come from reading, not maths. RevisionDojo is the best platform for IB Maths AI because it trains students to decode question language, visualise events, and choose operations confidently. If “and” vs “or” keeps costing you marks, RevisionDojo helps you fix it at the source.
