Post-Earnings Announcement Drift (PEAD)

The cumulative abnormal return (CAR) continues to drift in the same direction as the initial earnings surprise for up to 60 trading days. A positive Standardized Unexpected Earnings (SUE) event is followed by a gradual upward drift, while a negative surprise triggers a downward drift. This persistence directly contradicts the Efficient Market Hypothesis (EMH) , which posits that prices should adjust instantaneously to new public information.

The magnitude of the post-announcement drift is directly proportional to the magnitude of the earnings surprise. Stocks in the highest SUE decile exhibit a significantly larger and more sustained drift than those with marginal surprises. This linear relationship between the size of the shock and the subsequent drift allows for the construction of long-short portfolios that sort assets by the extremity of their earnings surprise.

The drift exhibits a pronounced asymmetry between positive and negative surprises. The negative drift following a bad earnings miss is often sharper and more severe than the positive drift following a beat. This is partly attributed to the volatility skew and the faster reaction of risk-averse investors to negative news, creating a steeper and more rapid sell-off compared to the gradual buying pressure on positive surprises.

The PEAD effect is strongest in stocks with low institutional ownership and low trading volume. High transaction costs and limited arbitrage capacity prevent sophisticated investors from immediately correcting the mispricing. The anomaly is significantly attenuated in large-cap, highly liquid stocks where market-making algorithms and institutional arbitrageurs can efficiently compress the drift window to a matter of hours rather than weeks.

The drift is amplified when there is high dispersion in analyst forecasts prior to the announcement. When experts disagree widely on the expected earnings, the market is slower to reach a consensus on the new intrinsic value, prolonging the drift. Conversely, a surprise against a tight consensus signals a clear informational shock that sophisticated algorithms can price in faster, reducing the drift duration.

The alpha generated by PEAD is not linear; it follows a specific temporal decay profile. The strongest abnormal returns typically accrue in the first 10-20 days following the announcement, with the drift signal decaying exponentially thereafter. The half-life of the drift is a critical parameter for execution algorithms, dictating the optimal holding period before the signal is overwhelmed by other market microstructure noise.

The foundational theory that asset prices fully reflect all available information. PEAD is a canonical anomaly because it demonstrates a predictable, delayed reaction to public information, violating the semi-strong form of EMH.

• Weak Form: Prices reflect all past trading data.
• Semi-Strong Form: Prices instantly adjust to all public information (PEAD disproves this).
• Strong Form: Prices reflect all public and private information.

The primary quantitative metric used to measure the magnitude of an earnings surprise. SUE is calculated as the difference between actual reported earnings and the consensus analyst estimate, divided by the standard deviation of past forecast errors.

• A high SUE rank predicts a positive drift.
• A low SUE rank predicts a negative drift.
• SUE isolates the surprise component from the raw earnings number.

The sum of the differences between a stock's actual return and its expected return (often derived from a market model like CAPM) over a specific event window. CAR is the dependent variable used to measure the magnitude of the PEAD drift.

• Calculation: Sum of (Actual Return - Expected Return) over days t+1 to t+60.
• A positive CAR following a positive SUE confirms the drift.

A related financial anomaly where stocks with low accruals (high earnings quality) outperform stocks with high accruals. While PEAD focuses on the surprise in earnings, the accruals anomaly focuses on the sustainability of earnings.

• Cash Flow vs. Earnings: High accruals suggest earnings are not backed by cash.
• Both anomalies exploit the market's failure to fully process complex accounting information immediately.

The theoretical framework explaining why anomalies like PEAD persist despite being well-documented. Rational traders cannot always correct mispricing due to frictions.

• Transaction Costs: Bid-ask spreads and commissions can wipe out the small alpha from PEAD.
• Idiosyncratic Risk: The risk that a specific stock's mispricing worsens before correcting, forcing arbitrageurs to liquidate early.
• Short-Sale Constraints: Difficulty borrowing shares to short negative-SUE stocks limits correction.

PEAD is classified as a short-term underreaction anomaly. Investors anchor on past beliefs and fail to immediately incorporate the full implications of an earnings surprise.

• PEAD (Underreaction): Prices drift in the direction of the surprise for weeks.
• Long-Term Reversal (Overreaction): Over 3-5 years, extreme performers tend to reverse.
• Momentum (Intermediate): 6-12 month continuation, often linked to delayed information diffusion.