Why Are Continuity Corrections So Easy to Forget in IB Maths?
Continuity corrections are one of the most common sources of small but costly mistakes in IB Mathematics: Analysis & Approaches. Many students understand binomial and normal distributions individually, yet still lose marks when asked to approximate one with the other. The issue is not lack of knowledge — it is forgetting what the approximation represents.
IB includes continuity corrections to test whether students truly understand the difference between discrete and continuous models. Missing a continuity correction often results in a correct method but an incorrect final probability.
What Is a Continuity Correction Actually Doing?
A continuity correction adjusts for the fact that the binomial distribution is discrete while the normal distribution is continuous.
When approximating a binomial distribution using a normal distribution, whole-number outcomes must be represented as intervals. The continuity correction expands the boundaries by 0.5 to ensure the continuous model captures the correct discrete outcomes. IB expects students to understand this adjustment conceptually, not just apply it mechanically.
Why Students Forget the ±0.5 Adjustment
Continuity corrections are easy to forget because they feel like an artificial rule added at the last moment.
Many students rush straight into standardisation and probability calculations without pausing to translate the original question correctly. IB examiners see many scripts where students use the correct mean and standard deviation but lose marks due to missing or incorrect boundary adjustments.
Why Direction Matters So Much
Another common mistake is applying the continuity correction in the wrong direction. Adding or subtracting 0.5 incorrectly shifts the probability region.
IB expects students to think carefully about whether they are finding “greater than,” “less than,” or “between” probabilities. Misinterpreting the direction of inequality often leads to the wrong shaded region under the curve, even when calculations are otherwise correct.
When Continuity Corrections Are Required — and When They Aren’t
Continuity corrections are required when:
- Approximating a binomial distribution with a normal distribution
- Calculating probabilities involving whole-number outcomes
They are not required when:
- Using the normal distribution directly
- Working purely with continuous models
IB frequently tests whether students know when the correction applies, not just how to use it.
Common Student Mistakes
Students frequently:
- Forget to apply the correction
- Apply it in the wrong direction
- Use it when it is not needed
- Standardise before correcting boundaries
- Shade the wrong region
Most errors occur at the interpretation stage, not during calculation.
Exam Tips for Continuity Corrections
Always write the original probability statement first. Translate discrete outcomes into continuous intervals before standardising. Sketch the distribution and label corrected boundaries. Check whether the correction is required at all. IB rewards careful setup just as much as correct arithmetic.
Frequently Asked Questions
Why does the correction use 0.5?
Because binomial outcomes increase in steps of 1. The correction shifts boundaries halfway between values so the continuous model matches discrete outcomes accurately. IB expects students to understand this logic.
Do I always lose marks if I forget the correction?
Often, yes. IB mark schemes typically penalise missing continuity corrections even if the rest of the method is correct. It is considered a conceptual error, not a minor slip.
Is it okay to skip explanation and just apply the correction?
Applying it correctly is usually sufficient, but understanding why helps prevent mistakes. IB values conceptual understanding, especially in explanation-heavy questions.
RevisionDojo Call to Action
Continuity corrections are easy to forget because they sit between discrete and continuous thinking. RevisionDojo helps IB students master this transition with clear explanations, visual reasoning, and exam-style practice. If small probability mistakes keep costing you marks, RevisionDojo is the best place to fix them.
