Expected number = row total x column total / grand total
X² = Σ(O-E)²/E
provided that the Os are frequencies and the Es are greater than 5
An (mxn) contingency table has (m-1)(n-1) degrees of freedom
If it is necessary to combine two classes to increase the size of the Es the most similar classes should be combined
For a 2x2 table: Σ(|O-E|-0.5)²/E
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