Correlation of two event indicators

Events AA and BB each have probability 0.50.5, and they co-occur with P(AB)=0.4P(A\cap B)=0.4. What is the correlation between their indicator variables 1A,1B\mathbf{1}_A,\mathbf{1}_B? Give a decimal to two places.

Show hints (2)+
  1. Covariance first: P(AB)P(A)P(B)P(A\cap B)-P(A)P(B).
  2. Each indicator's variance is p(1p)p(1-p), not pp; divide the covariance by VarAVarB\sqrt{\operatorname{Var}_A\operatorname{Var}_B}.

Answer

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0.6 (± 0.001)

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