Corporate Actions in Backtests: Splits, Dividends, Mergers

Corporate Actions, Delistings and Rolls in Backtests

Splits, dividends, mergers, spin offs and delistings change prices and holdings. Learn how each affects backtests, how to adjust data and the errors to avoid.

Advanced3 min readUpdated 3 Oct 2026

Lesson 20 of 38

Corporate actions are events that change a company's shares or pay out value to shareholders: stock splits, dividends, mergers, spin offs, rights issues and delistings. If a backtest ignores them, prices appear to jump or crash for no reason, dividends vanish from returns and holdings disappear without explanation. Getting corporate actions right is essential for any equity backtest, especially over long periods and across many stocks.

The main corporate actions#

Action	Effect on raw prices	Backtest treatment	Lesson
Stock split	Price falls by the split ratio	Adjust past prices or share counts	Stock Splits
Reverse split	Price rises by the ratio	Adjust similarly	Stock Splits
Cash dividend	Price drops by about the dividend on the ex date	Add dividends to returns or use total return prices	Dividends
Special dividend	Larger drop	Same, with care	Dividends
Spin off	Parent price drops; new shares distributed	Add the new company's shares to holdings	Spin-Offs
Merger (cash)	Shares cancelled for cash	Convert holding to cash at deal price	Mergers and Acquisitions
Merger (stock)	Shares converted into acquirer shares	Convert holding at the exchange ratio	Mergers and Acquisitions
Rights issue	New shares offered at a discount	Adjust prices for the value of rights
Delisting or bankruptcy	Trading stops	Apply final return, which may be near minus 100%	Bankruptcy and Restructuring

Adjusted vs unadjusted prices#

Price type	Use
Unadjusted (raw)	What actually traded on each day; needed for order sizes, limit prices and liquidity filters
Split adjusted	Removes split jumps; good for charts and price based signals
Total return adjusted	Removes splits and reinvests dividends; good for return calculations

See Splits and Dividends in Price Data.

Common errors#

Error	Consequence
Unadjusted splits	False crashes trigger stops and sell signals
Using adjusted prices for share counts	Wrong position sizes and liquidity estimates
Ignoring dividends	Understated returns; distorted value and income strategies
Mishandling spin offs	Apparent large losses in the parent
Dropping delisted stocks	Survivorship bias. See Survivorship and Selection Bias
Back adjustment that changes past signals	Look ahead effects when adjustments use future events

Back adjustment and look ahead#

Adjusted price series are recalculated whenever a new corporate action occurs, so historical adjusted prices change over time. For most signals based on returns, this is harmless. But rules based on absolute price levels, such as "stocks under $5", should use the raw price known at the time. See Look-Ahead Bias and Point-in-Time and Survivorship-Free Data.

Practical approach#

Store raw prices and a separate corporate actions table.
Compute returns with total return adjustments.
Use raw prices for order sizing, price thresholds and liquidity filters.
Process events on their effective dates in portfolio backtests.
Include delisting returns from a reliable source.
Cross check a sample of events by hand.

Corporate actions in other markets#

Futures: contract rolls play a similar role. See Continuous Futures and Back-Adjustment.
ETFs: distributions and splits.
Crypto: token migrations, redenominations and forks can require similar adjustments.

Frequently asked questions#

What are corporate actions?#

Company events that change shares or pay value to shareholders, such as splits, dividends, mergers, spin offs and delistings.

Why do corporate actions matter in backtests?#

Because unadjusted data shows false price jumps, misses dividend income and mishandles holdings, distorting results.

Should I use adjusted or unadjusted prices?#

Use total return adjusted prices for return calculations and unadjusted prices for order sizes, price thresholds and liquidity checks.

Next, learn to make your research repeatable in Backtest Reproducibility.

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