Summary OCR A-Level Biology 6.2.9 Chi-Squared Test

Chi-Squared Test
( O−E)²
 Formula: χ² = ∑
E
 E = Expected Number
 O = Observed Number
 Degrees of Freedom = Number of Classes – 1 (N-1)
 Significance Level = 5% = 0.05
 Null Hypothesis = There is no significant difference between the observed and expected
date at the 5% significance level, any difference is due to chance
 If χ² > CV, difference is significant, reject null hypothesis
 If χ² < CV, difference is due to chance, accept null hypothesis



Example


Category Observed (O) Expected (E) (O-E)² (O−E)²
E
Yellow, Round 169 162 49 0.30
Green, Round 54 54 0 0.00
Yellow, Bumpy 51 54 9 0.17
Green, Bumpy 14 18 16 0.88
χ² 1.35


 Degrees of Freedom = 4 - 1 = 3
 CV = 7.82
 1.35 < 7.82
 χ² < CV
 Accept null hypothesis, difference is due to chance