The Paradox of the Pre-Trade Cost Model

Robert Almgren – The most important consequence is the impossibility of distinguishing market impact from alpha. The trader sends a buy order because he or she anticipates that the market will rise; if it rises during execution of the buy order, was that because the execution pushed the market, or because the trader correctly anticipated the move? It is impossible to distinguish these two effects. Our model simply reports the combination of the two, assuming that whatever combination of alpha and impact prevailed in the past will continue to hold during future executions.

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