hard · Market Microstructure

Consider the PIN (Probability of Informed Trading) model of Easley-O'Hara: information events occur with probability α, are bad news with probability δ, informed traders arrive at rate μ on event days, and uninformed buyers and sellers arrive at rates varepsilon_b and varepsilon_s. A researcher computes PIN = αμ / (αμ + varepsilon_b + varepsilon_s) and uses it as a measure of adverse selection. Two stocks have IDENTICAL estimated PIN, but Stock A has high α with low μ while Stock B has low α with high μ.

Which conclusion about the implied price impact of an unexpected buy order is correct?

  1. Both stocks have the same unconditional PIN but Stock B's order imbalance is more diagnostic on event days, so a single unexpected buy moves Stock B's price more despite equal PIN.
  2. Equal PIN implies equal price impact for any order by construction, since PIN is the sufficient statistic for adverse-selection price response.
  3. Stock A has higher price impact because its higher event frequency α means any given order is more likely to occur on an information day.
  4. Neither order has price impact because the unconditional buy probability already incorporates the informed component, leaving the posterior unchanged.

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