Chi-squared Test
A hypothesis test using the test statistic χ² = Σ((O-E)²/E) where O represents observed frequencies and E represents expected frequencies. It tests the null hypothesis that two categorical variables are independent. The test statistic follows an approximate chi-squared distribution with appropriate degrees of freedom.
Real World
The UK government's Road Safety Analysis team uses chi-squared tests to investigate whether accident severity (slight, serious, fatal) is associated with road type (motorway, A-road, residential), helping to prioritise safety interventions.
Exam Focus
State H₀ as 'the two variables are independent' and H₁ as 'the two variables are not independent' — marks are awarded for precise wording.
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