Market-order slippage on split-adjusted stock prices

How the price series is constructed

Algorithmic strategies can be applied to stocks, futures, foreign exchange, options, and cryptocurrencies, but each venue has distinct data and execution details that change how costs should be measured.

Three equity market-data treatments can materially distort a historical test if they are handled incorrectly, and all three concern how the price series is constructed rather than the signal itself.

What a stock split does to the quotes

A stock split is a corporate action that increases share count and lowers the quoted price. After the split, data vendors typically rewrite earlier prices so the series does not show a gap, which changes the level of the historical quotes used for cost and slippage estimates.

The split-adjusted price is a restated historical quote that removes split gaps so the series looks continuous. It is not the cash price a contemporaneous order would have paid. A split-adjusted series can print well below one currency unit, which can make a past 100-share purchase look like it cost only a few tens of currency units when that fill never existed at the adjusted price.

When a live bid-ask meets a split-adjusted print

A market order is an instruction to trade immediately against displayed liquidity, so the fill inherits the bid-ask spread and any slippage. The bid-ask spread is the gap between the best bid and the best offer and is a primary component of market-order implementation cost. Slippage is the difference between an assumed or decision price and the price obtained when the order is filled.

A live large-cap bid-ask near 0.01 per share is a small fraction of the unadjusted price, but applying that same absolute market-order slippage to an adjusted print of 0.35 implies a move of 3 percent or more just to cover the cost.

Dividend adjustment and the executable order

Dividend adjustments are another corporate-action rewrite that can change the historical prices fed into a transaction-cost or slippage simulation. A dividend adjustment rewrites the historical series to reflect cash distributions, which also changes the levels used in cost tests.

Choosing an executable market order and measuring implementation cost requires knowing how splits and dividends were treated in the historical series. Otherwise a cost simulation applies a current tick-size assumption to a non-tradable adjusted price. Implementation cost is the full cost of turning a signal into a fill, including spread, slippage, and other frictions over the order's life.

Commission analysis accounts for explicit fees together with implicit execution costs when judging whether a candidate fill would have been tradable. A transaction-cost and slippage simulation is a historical check that applies spread or slippage assumptions to candidate market orders before they are treated as executable.