Chi-squared questions appear almost every year in IB Math AI SL, yet many students still lose easy marks.
Not because the maths is impossible. Usually because they do not answer the question the way IB expects.
Step 1: State the hypotheses
You should always write:
- H0: the variables are independent
- H1: the variables are dependent
Step 2: Calculate expected values
Use the expected value formula carefully. This is where setup matters more than speed.
Expected value formula
Expected = (row total × column total) / grand total
Step 3: Calculate χ²
Once your expected values are correct, calculate the chi-squared test statistic cleanly and keep the working visible.
Chi-squared test statistic
χ² = Σ((O − E)² / E)
Step 4: Compare values
- Compare χ² with the critical value, or
- Use the p-value if the question is set up that way
Do not stop at the calculator screen. The comparison only matters if you use it to make a clear decision.
Step 5: Write the conclusion
This is the most important part of the whole question.
Weak answer
Reject H0.
Full-mark answer
Reject H0 — there is sufficient evidence that the variables are dependent.
Quick reality check: many students know the formula but still lose the question because the final line is too vague.
Common mistakes
- No conclusion in context
- Wrong hypotheses
- Only doing the calculation
- Rounding too early
Final tip
Chi-squared is one of the easiest marks in IB Math AI SL if you learn the structure properly.
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