Corporate Actions: Adjusted Prices Are a Modeling Choice
Splits, dividends and spin-offs change the price series; how they are applied decides what the backtest measures.
In this note
Working definition
A corporate action is an issuer event, such as a split, dividend, spin-off, merger or symbol change, that alters shares outstanding, cash paid to holders, or the instrument itself. Adjusted prices restate history to remove the mechanical effect of these events.
01
Returns and prices answer different questions
A two-for-one split halves the share price without changing the value of a holding. An unadjusted series shows a 50% drop; a split-adjusted series removes it. Dividends work differently: the price typically falls by about the amount paid, while the holder receives cash. A price-only series misses that income and a total-return series reinvests it. Each is correct for a different question.
Signals and accounting can need different series. A momentum signal usually wants returns that include dividends, so that a stock is not penalized simply for paying out cash. A rule that filters on price levels, for example one that excludes stocks below five dollars, needs the price that traded on that day, not a level rescaled by later splits.
02
Back-adjustment rewrites the past
Many data sources back-adjust: when a split or dividend occurs, all earlier prices are rescaled. The adjusted history therefore changes after every new event, and a value stored today may not match the value retrieved next year. That is harmless for return calculations and risky for anything that uses price levels, volumes or round-number thresholds.
Order sizes are a common casualty. Share counts derived from back-adjusted prices do not match the shares that traded, and liquidity filters based on adjusted volume can admit or reject the wrong names. Keep unadjusted prices and volumes for execution and capacity, and adjustment factors with their effective dates for returns.
- Store raw prices, raw volumes and dated adjustment factors separately.
- Measure performance on total returns and apply price-level rules to the prices that traded.
- Handle special dividends, spin-offs and mergers explicitly rather than through a single factor.
03
Events that change the instrument
Spin-offs, mergers and ticker changes are harder than splits. A holder of the parent may receive shares of a new company; an acquired stock may be converted into cash or into the acquirer's shares. A backtest that silently drops the position at the event date, or treats a renamed ticker as a new listing, misstates both returns and the universe.
A reviewer should be able to trace a sample of events end to end: the announcement, the effective date, the adjustment applied, and the position before and after. Where the data cannot support that trace, the affected names should be identified and their contribution to results reported.
Practical takeaways
- Split-adjusted, dividend-adjusted and unadjusted prices answer different questions.
- Back-adjusted history changes after each new event; store raw values and dated factors.
- Use traded prices and volumes for order sizes, filters and capacity.
- Trace spin-offs, mergers and symbol changes explicitly instead of letting positions vanish.
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