Market Impact

Market impact is the movement in the price of a security that is caused by a trader's own buying or selling. A large buy order pushes the price up as it consumes the offers available in the market, and a large sell order pushes it down, so the average price achieved is worse than the price that was quoted when the decision to trade was made. For institutional investors that trade in size, market impact is usually the largest component of trading cost, greater than commissions or fees.

The concept belongs to the study of market microstructure and to the practice of transaction cost analysis. It is related to, but not the same as, slippage, which is the general difference between an expected and an achieved price and also includes price moves that had nothing to do with the order. Market impact is the part of that difference that the order itself caused.

Two mechanisms are at work. The first is mechanical. At any moment the order book holds a limited quantity at each price, and an order larger than the quantity at the best price must trade at successively worse prices. The thinner the book, measured by order book depth, the further a given order walks through it.

The second is informational. Other participants cannot tell whether a large buyer knows something about the value of the security, so they treat sustained buying as a signal and raise their quotes. A market maker who has just sold to a persistent buyer will also move prices up to rebuild inventory. Both effects mean that the price does not simply bounce back when the order is finished.

Analysts split impact into two parts. Temporary impact is the price concession needed to find liquidity quickly; it fades once the trading pressure stops and other participants replenish the book. Permanent impact is the lasting shift in the price that remains after the order is complete, reflecting the information the market inferred from it.

The distinction matters for trading strategy. Temporary impact rises with the speed of execution, so it can be reduced by trading more slowly. Permanent impact depends mainly on the total size of the order relative to normal trading volume and cannot be avoided by slicing, although it can be shared with a willing counterparty in a block trade.

Impact cannot be observed directly, because nobody sees the price that would have prevailed had the order not been sent. It is estimated by comparing execution prices with benchmarks. The most common are the price at the time the order arrived, used in implementation shortfall, and the volume-weighted average price over the trading period.

Empirical studies across many markets have found that impact grows less than proportionally with order size. A widely used rule of thumb, often called the square-root law, estimates impact as proportional to the security's volatility multiplied by the square root of the order's size as a share of daily volume. Under this rule, quadrupling an order roughly doubles its impact. Optimal execution models, of which the Almgren and Chriss framework published around 2000 is the best known, use impact estimates to schedule an order over time, weighing the cost of trading fast against the risk that the price drifts while trading slowly.

Institutional desks use several techniques to limit the footprint of their orders:

• Order slicing. A parent order is divided into many small child orders spread over hours or days, usually by an algorithmic trading strategy that follows a volume or time schedule.
• Hidden liquidity. Orders are sent to venues that do not display them before execution, such as dark pools, or are entered as iceberg orders that show only part of their size.
• Auctions. Trading in the opening or closing auction concentrates liquidity at a single price and lets large orders be absorbed with less disturbance.
• Negotiated trades. A broker commits capital or finds a natural counterparty, and the trade is executed off the book at an agreed price.

Each technique involves a trade-off. Slower execution reduces temporary impact but leaves the investor exposed to market moves for longer, and information about a large order can leak whichever route is chosen.

Market impact is relevant beyond trading desks. Best execution rules require brokers to consider total cost, including implicit costs, when handling client orders. Fund managers must consider how the cost of dealing in portfolio assets is shared among investors, which is the rationale for swing pricing. Risk managers apply impact estimates when judging how quickly a position could be liquidated in stress, and regulators consider it when assessing liquidity risk in funds and trading books. Deliberately using the impact of one's own orders to move a price for gain elsewhere can amount to market manipulation.