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How do you interpret a Bayes factor?

hard

Answer

  • BF12  =  p(D    M1)  /  p(D    M2)\mathrm{BF}_{12}\; = \;p(D\; \mid \;M_{1})\; / \;p(D\; \mid \;M_{2}) — ratio of marginal likelihoods → posterior odds = BF × prior odds.
  • Jeffreys scale: BF > 10 strong evidence for M1M_{1}, > 100 decisive; BF < 1/10 flip.
  • Unlike p-values, symmetric — can support H0H_{0}.
  • Weakness: hypersensitive to prior choice for nuisance parameters (Lindley's paradox).
  • Alternative in practice: LOO-CV, WAIC, PSIS-LOO for predictive model comparison.
Check yourself — multiple choice
  • Random
  • Ratio of marginal likelihoods; Jeffreys scale > 10 strong, > 100 decisive; symmetric (supports  H0)(\mathrm{supports}\;H_{0}); prior-sensitive → also use LOO-CV / WAIC
  • Same as p-value
  • Not useful

Bayes factor: p(DM1)/p(DM2)p(D \mid \mathrm{M1}) / p(D \mid \mathrm{M2}); symmetric; prior-sensitive.

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