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To illustrate the losses from market impact, suppose XYZ stock has a “true” price of $50 a share. Assuming for simplicity that it trades in 10-cent increments, between trades there will be buyers bidding for various amounts at $49.90, $49.80, $49.70, and so forth. Similarly, sellers will be asking $50.10, $50.20, et cetera. Someone who places an order to buy at whatever price is available in the market, called a market order and one of the most common types, will pay $50.10, a little above the true price. This 10-cent difference between the price paid and the “true” price is called market impact. Market impact increases with order size since, to continue our example, a large market order may clean out not only the offering at $50.10 but also stock offered for sale at higher prices, resulting in an average purchase price above $50.10 and a market impact greater than 10 cents per share. When Steve Mizusawa and I operated Ridgeline Partners, we reduced these costs by dividing large orders into smaller ones of $20,000 to $100,000, and waiting a few minutes between transactions to allow the market price to recover. We know the “true” price is somewhere at or between the highest bid price (the Bid) and the lowest asking price (the Offer), but not exactly where. On average, it is about halfway between the two. To see that market impact is a real cost, suppose in our example that just after buying stock at $50.10 the buyer wants to sell it at market. He gets $49.90, for an immediate loss of 20 cents or about 0.4 percent.
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Edward O. Thorp (A Man for All Markets: From Las Vegas to Wall Street, How I Beat the Dealer and the Market)