MYP Maths Criterion C: Communicating — FAQ

Like every MYP subject criterion, Criterion C is marked from 0 to 8 across four achievement bands (1-2, 3-4, 5-6, 7-8), each with its own level descriptors set out in the IB's MYP Mathematics guide. Two students can reach the identical numerical answer and land in different bands — the mark follows the explanation, not the result.

Breaking it down:

1. Language — correct notation and vocabulary used throughout, not just in a final sentence.
2. Representation — choosing forms (graph, table, diagram, equation) that actually suit the problem.
3. Movement between forms — explicitly connecting them with reasoning.

In my marking, strand iii is where most level 5-6 students stall. They'll produce a graph and an equation but never write the sentence tying the two together — and that missing sentence is usually the whole gap to 7-8.

Worked example — modelling a ball's height h (metres) after t seconds using h = -5t² + 20t + 1:

Level 3-4 response: "t = 2, h = 21. Max height 21m." No working, no labels, no context.

Level 7-8 response: "Since the coefficient of t² is negative, the graph is a downward parabola and its maximum occurs at the vertex. Using t = -b/2a = -20/(2×-5) = 2 seconds. Substituting: h(2) = -5(2)² + 20(2) + 1 = 21. This matches the turning point shown on the graph at (2, 21), so the ball reaches a maximum height of 21 metres after 2 seconds."

5 things to check before submitting:

1. Correct notation throughout, not just the final line.
2. At least two representations included.
3. Full sentences linking them.
4. Would someone a term behind you follow it unaided?
5. A conclusion stated back in context, not just a bare number.

Quick reference by topic:

• Algebra: coefficient, expand, factorise, simplify, solve
• Graphs: gradient, intercept, asymptote, turning point, domain/range
• Statistics: correlation, outlier, distribution, mean vs median
• Geometry: congruent, similar, bisector, subtend

Quick tip: if a command term in the task is "justify" or "explain", a purely numerical answer — however correct — will not access the top band on its own.

Quick tip: don't just place a graph next to your algebra and hope the examiner joins the dots. Write the linking sentence yourself — something like "this confirms the x-intercept found algebraically at x = 3" — because that sentence, not the graph, is what strand iii is actually marking.

Think of it this way: Criterion A checks the maths is right. Criterion C checks the maths is understandable. A script can score full marks on A with messy, unlabelled working, and still sit in a low Criterion C band because none of that working was communicated clearly.

Common mistake: students assume a well-formatted, neatly explained solution automatically scores well on both criteria. It doesn't — a clearly communicated model that ignores real-world constraints (negative time, impossible dimensions) can still lose marks on Criterion D even while scoring 7-8 on C.

Common mistake: skipping steps because "it's obvious". It might be obvious to you mid-solution; it isn't obvious to the person marking it against level descriptors. Write the step anyway.

If a test question starts with "explain why", "justify", or "describe the relationship", that's a Criterion C-style question even inside a timed assessment — treat it differently from a pure calculation question.

See the comparison table below for what each criterion actually rewards and where it typically gets assessed.

The IB's standard MYP grade-boundary conversion (used across all subject groups, from the four-criteria total out of 32) is:

Schools may adjust rubrics locally, but this is the model most use — so a weak Criterion C score (say 3/8 against strong 7s elsewhere) can pull a total from a 6 down into a 5.

3 questions to ask over their shoulder:

1. "Which two ways have you shown this — can you show me a second one?"
2. "Can you read that working aloud as if I've never seen the problem?"
3. "Where's the sentence that says what the answer actually means?"

These three questions map almost exactly onto the three Criterion C strands.

When practising, mark your own attempt against the achievement-level bands rather than just checking the final number — that's the habit that actually mirrors how a teacher scores Criterion C, and it's a faster way to spot missing representations or unclear reasoning than waiting for feedback.