When Is Interpolation Actually Valid in IB Maths?
Interpolation usually feels safer than extrapolation for IB Mathematics: Applications & Interpretation students. Because it stays within the range of known data, students often assume interpolation is always acceptable and reliable. However, IB still expects judgement — interpolation can be inappropriate if the underlying assumptions are ignored.
IB tests interpolation to assess whether students understand what the data represents, not just where the numbers sit on a graph.
What Interpolation Really Means
Interpolation estimates values between known data points.
It assumes that the behaviour between those points follows a reasonably smooth and predictable pattern. IB expects students to recognise that interpolation is only valid if this assumption makes sense in context.
Interpolation is safer than extrapolation, but it is not automatic.
Why Interpolation Is Often Valid
Interpolation works best when:
- Data changes smoothly
- There are no sudden jumps or thresholds
- The context supports gradual change
In many applied settings — such as temperature trends, steady financial growth, or gradual population changes — interpolation is reasonable. IB often expects students to use it in these situations.
When Interpolation Becomes Unreliable
Interpolation is not valid if the relationship between data points is irregular or discontinuous.
Examples include:
- Situations with sudden events
- Threshold-based systems
- Step changes or caps
- Discrete outcomes misread as continuous
IB expects students to think about whether intermediate values actually make sense, not just whether they lie between two numbers.
