What is post-earnings announcement drift (PEAD), and why is it considered one of the most robust market anomalies?

Question

AcadiFi · Accepted Answer

PEAD is arguably the most well-documented anomaly in empirical finance. It was first identified by Ball and Brown in 1968 and has persisted across markets, time periods, and methodologies.

**The Phenomenon**

After a positive earnings surprise, stock prices continue to drift upward for 60-90 trading days. After a negative surprise, prices continue to drift downward. The magnitude of the drift correlates with the size of the standardized unexpected earnings (SUE).

**How SUE Is Calculated**

SUE = (Actual EPS - Expected EPS) / Standard Deviation of Past Forecast Errors

Higher SUE values correspond to more pronounced drift.

**Worked Example — Corwin Pharmaceuticals:**

Corwin reports Q2 EPS of $2.18 vs. consensus estimate of $1.85. Historical forecast error standard deviation is $0.11.

SUE = ($2.18 - $1.85) / $0.11 = **3.0**

This is a very large positive surprise. Empirical evidence suggests that top-decile SUE stocks earn approximately 2-4% in abnormal returns over the following 60 days as the market gradually incorporates the surprise.

**Why Does PEAD Persist?**

Several explanations have been proposed:

1. **Investor underreaction** — anchoring bias causes investors to under-adjust their expectations after surprises
2. **Transaction costs** — the drift may be too small relative to trading costs for large institutions to profitably exploit
3. **Risk-based explanations** — perhaps the drift compensates for bearing some unmeasured risk (though evidence for this is weak)
4. **Limits to arbitrage** — short-selling constraints prevent rapid correction after negative surprises

**Trading Strategy Implications:**

A PEAD strategy involves going long stocks with high positive SUE and shorting stocks with high negative SUE, rebalancing after each earnings season. Academic studies show long-only PEAD portfolios have historically earned 4-7% annualized alpha, though returns have compressed in recent decades as more investors became aware of the anomaly.

**Key Exam Points:**

1. PEAD directly challenges semi-strong efficiency since earnings announcements are public information.
2. The drift is stronger for smaller, less liquid stocks with lower analyst coverage.
3. PEAD has been documented in international markets, suggesting it is not a U.S.-specific data artifact.
4. The anomaly has weakened but not disappeared — consistent with partial market learning.

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What is post-earnings announcement drift (PEAD), and why is it considered one of the most robust market anomalies?

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