Day trading strategy: intraday momentum

A recent study called Market Intraday Momentum by Lei Gao, Yufeng Han, Sophia Li, and Guofo Zhou sheds light on a powerful, often overlooked phenomenon in financial markets: intraday momentum.

This research reveals that the market's performance in the first half-hour of trading can significantly predict its direction in the last half-hour of the very same day.

For anyone keen on market dynamics, this finding offers value into short-term predictability and potential economic value.

First Half-Hour Predicts Last

The study, which analyzed high-frequency data for the S&P 500 ETF (SPY) from 1993 to 2013, pinpointed a robust positive correlation.

Specifically, the return from the previous day's close to 10:00 AM Eastern Time (the "first half-hour return") often predicts the return from 3:30 PM to 4:00 PM Eastern Time (the "last half-hour return").

• This is a nice contribution, as most prior momentum research has focused on monthly or weekly patterns across different assets, not within a single trading day.

• The first and last half-hours are particularly active, exhibiting a "U-shaped" pattern in trading volume and volatility. The initial half-hour processes pre-market news, while the final half-hour sees significant institutional trading for portfolio valuation and avoiding overnight risk.

What This Means for Investors: Real Economic Value

The predictability isn't just academic; it translates into profits.

• Market Timing Strategy: A simple strategy, going long if the first half-hour return is positive and short if negative, yielded an average annualized return of 6.67%. This significantly outperforms a daily Buy-and-Hold strategy (6.04% annualized return with a much higher standard deviation). The timing strategy also boasts a remarkable Sharpe ratio of 1.08, compared to 0.29 for Buy-and-Hold.

• Utility Gains: For a mean-variance investor with typical risk aversion, this predictability can generate extra risk-adjusted returns of 6.02% per annum.

When Intraday Momentum Is Strongest

The study identified several conditions that amplify this predictable pattern:

• High Volatility Days: The predictability intensifies significantly on days with higher initial volatility.

• Higher Trading Volume Days: Similar to volatility, more trading volume in the first half-hour correlates with stronger momentum.

• Recession Days: Intraday momentum is considerably more potent during economic recessions than expansions.

• Major Macroeconomic News Days: Days with key news releases, such as GDP, CPI, and especially Federal Open Market Committee (FOMC) minutes, show dramatically stronger momentum. On FOMC release days, a timing strategy delivered an astounding 20.04% annualized average return.

Broad Impact: Beyond Just the S&P 500

This isn't just an SPY-specific anomaly. The study confirms significant intraday momentum across ten other actively traded domestic and international ETFs. This includes indices like the Dow, NASDAQ, and Russell 2000, as well as sector-specific and international equity ETFs.

In some cases, the predictability and economic gains (CERs) were even greater than for SPY, possibly due to lower liquidity in these other ETFs.

Furthermore, the profitability of exploiting this momentum persists even after accounting for transaction costs, especially in the post-decimalization era (after 2001) where spreads narrowed significantly.