negative cost

What does ‘negative cost’ under implementation shortfall means?

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For example, between the time you decide to buy and the time you actually buy, the price falls.

think of negative costs as the benefit you have when waiting to trade at a more advantageous price.

For instance, the current price when you decide to buy a stock is 50 (which is also your benchmark). You take a limit order position to buy the stock at 49, which is below the market price. If the price falls to your limit order, then you would gain a negative cost(benefit) from buying the stock at 49 instead of 50.

Thank you