MYP · Mathematics MYP Year 3
Probability and Outcomes
24 min readStandardCore strand of the Statistics & Probability unit — assessed through unit tests and MYP Criterion A/D investigation tasks; recurs in every subsequent probability unit as the foundation for compound and conditional work.
Probability is careful counting dressed up as prediction. Every question in this topic reduces to comparing two numbers: the theoretical fraction you calculate before you roll, spin or draw, and the experimental fraction you calculate after. Get comfortable listing outcomes without missing any — the rest (trees, grids, Venn diagrams) is just bookkeeping around that one skill.
Overview
This chapter has exactly two families of question: those asking you to predict a probability from counting outcomes (theoretical), and those asking you to measure one from recorded data (experimental). Everything else — dice, coins, spinners, trees, Venn diagrams, sample space grids — is just the setting the counting happens in.
- The probability scale (0 to 1) gives you the vocabulary: impossible, unlikely, even chance, likely, certain.
- Single events (one die roll) become combined events (two dice, spinner + die) once you're tracking more than one action at once.
- Listing outcomes systematically — as a set, a grid, or a tree — is the skill every later calculation depends on.
The shape of the chapter
Command terms this topic actually uses
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Calculate | Obtain a numeric answer, showing the working/formula used. | Knowing and applying | Mark is for the correct method AND the final value — a bare answer with no formula shown can lose the method mark. |
| Explain | Give a reason connected to the maths, not just restate the situation. | Communicating | Vague answers ('it's rare', 'it's not fair') earn nothing without a specific mathematical or physical reason. |
| Construct | Build the actual diagram or table completely and correctly. | Applying maths | Marked on structure/completeness — one missing cell or branch loses marks even if later arithmetic is fine. |
| Compare | Put two values in the same format and state which is bigger and by roughly how much. | Communicating | Comparing raw counts instead of probabilities (e.g. '6 blue > 3 blue') scores zero — must normalise first. |
| Justify | Give a clear verdict (correct/incorrect, true/false) backed by a calculated value. | Reasoning | A calculation with no stated verdict, or a verdict with no calculation, only earns partial credit. |
| Apply | Use a given formula on new numbers. | Knowing and applying | Marks for correct substitution into the stated formula — using a different (unstated) method can cost marks even if numerically correct. |
| State | Give a fact directly, no working required. | Knowledge | No explanation expected or rewarded — but the value must still be exact (e.g. 1/6, not 0.17). |
Key point
Every probability answer is a number between 0 and 1 (or 0% and 100%) — never negative, never above 1. If your answer breaks that rule, you've miscounted the sample space or mixed up favourable/total.
Overview