# 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.

Source: https://learn.tradelabsai.com/research/corporate-actions-in-backtests/  
Track: Research and Backtesting · Level: Advanced · Updated: 2026-10-03  
Publisher: TradeLabs AI (https://tradelabsai.com). Education, not financial advice.  
Cite as: TradeLabs Learn, "Corporate Actions, Delistings and Rolls in Backtests", https://learn.tradelabsai.com/research/corporate-actions-in-backtests/

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](https://learn.tradelabsai.com/fundamentals/stock-splits/) |
| Reverse split | Price rises by the ratio | Adjust similarly | [Stock Splits](https://learn.tradelabsai.com/fundamentals/stock-splits/) |
| Cash dividend | Price drops by about the dividend on the ex date | Add dividends to returns or use total return prices | [Dividends](https://learn.tradelabsai.com/fundamentals/dividends/) |
| Special dividend | Larger drop | Same, with care | [Dividends](https://learn.tradelabsai.com/fundamentals/dividends/) |
| Spin off | Parent price drops; new shares distributed | Add the new company's shares to holdings | [Spin-Offs](https://learn.tradelabsai.com/fundamentals/spin-offs/) |
| Merger (cash) | Shares cancelled for cash | Convert holding to cash at deal price | [Mergers and Acquisitions](https://learn.tradelabsai.com/fundamentals/mergers-and-acquisitions/) |
| Merger (stock) | Shares converted into acquirer shares | Convert holding at the exchange ratio | [Mergers and Acquisitions](https://learn.tradelabsai.com/fundamentals/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](https://learn.tradelabsai.com/fundamentals/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](https://learn.tradelabsai.com/programming/adjusted-prices/).

**Example: Dividends matter**
Over 20 years, a stock's price rises from $40 to $100, a price return of 150%. It also paid dividends averaging 3% a year. Reinvesting them, the total return is roughly 350%. A backtest using price only would understate returns and misjudge strategies that favour high dividend stocks. Conversely, a strategy that sells just before ex dates would wrongly appear to avoid price drops if dividends were ignored. See [Free Cash Flow Yield and Dividend Yield](https://learn.tradelabsai.com/fundamentals/dividend-yield/).

## 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](https://learn.tradelabsai.com/research/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](https://learn.tradelabsai.com/research/look-ahead-bias/) and [Point-in-Time and Survivorship-Free Data](https://learn.tradelabsai.com/programming/point-in-time-data/).

## Practical approach

1. **Store raw prices and a separate corporate actions table.**
2. **Compute returns with total return adjustments.**
3. **Use raw prices** for order sizing, price thresholds and liquidity filters.
4. **Process events on their effective dates** in portfolio backtests.
5. **Include delisting returns** from a reliable source.
6. **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](https://learn.tradelabsai.com/futures/continuous-futures/).
- **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](https://learn.tradelabsai.com/research/backtest-reproducibility/).

## Continue learning

- Next lesson: [Backtest Reproducibility](https://learn.tradelabsai.com/research/backtest-reproducibility/)
- Previous lesson: [Portfolio and Multi-Asset Backtesting](https://learn.tradelabsai.com/research/portfolio-backtesting/)
- Related: [Portfolio and Multi-Asset Backtesting](https://learn.tradelabsai.com/research/portfolio-backtesting/): Portfolio backtests simulate many positions with capital limits, sizing and rebalancing. Learn the key design choices, constraints, metrics and pitfalls.
- Related: [Splits and Dividends in Price Data](https://learn.tradelabsai.com/programming/adjusted-prices/): Adjusted prices remove the jumps caused by splits and dividends so returns are correct. Learn how adjustment factors work, when to use raw prices and common traps.
- Related: [Stock Splits](https://learn.tradelabsai.com/fundamentals/stock-splits/): A stock split increases share count and lowers price without changing company value. Learn how splits and reverse splits work, why they happen and the reaction.
- Related: [Dividends](https://learn.tradelabsai.com/fundamentals/dividends/): Dividends are cash payments companies make to shareholders. Learn the key dates, types of dividends, dividend policy, taxes and their effect on prices and options.
- Related: [Spin-Offs](https://learn.tradelabsai.com/fundamentals/spin-offs/): A spin off separates a business into a new listed company owned by existing shareholders. Learn how spin offs work, why they happen, forced selling and the evidence.
- Related: [Survivorship and Selection Bias](https://learn.tradelabsai.com/research/survivorship-and-selection-bias/): Survivorship bias ignores failures; selection bias picks unrepresentative samples. Learn how both inflate backtests, fund returns and advice, and how to fix them.