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APT:

Asset Returns explained by factor models
Asset specific risk can be diversified away
Assets are priced so that there are no arbitrage possibilities

APT is an Equilibrium Pricing Model
Indicates what Er should be under no arb assumptions
Macroeconomic Multifactor models are adhoc
Multifactor return models are time series regression models that explains variation in one asset
Intercept return in Macro model= assets expected return (not Risk free rate like APT or Capm)
Mulitfactor models, factors represent "surprises" vs APT where factors are risk premia.