Performance & Risk Metrics
Market Beta Drift
The systematic change in a security or portfolio's sensitivity to broad market movements over time, indicating shifting exposure to systematic risk.
Market beta drift is the tendency of a stock's sensitivity to the market to change over time rather than stay fixed. A company that takes on leverage, shifts its business mix, or moves between growth and value regimes will see its beta move with it. Treating beta as a constant when it is in fact drifting leads to mispriced hedges and misattributed alpha.
You expose drift by re-estimating beta over rolling windows (commonly 30, 60, or 90 days) and watching how the estimate moves. A short window reacts fast but is noisy; a long window is smooth but lags real change. Comparing a stock's beta path to its sector peers helps tell a genuine structural shift from ordinary estimation noise.
Formula
- \betat
- Beta estimated at time t over rolling window
- Rasset,t
- Asset return at time t
- Rmarket,t
- Market benchmark return at time t
- \tau
- Lookback period for drift comparison (days or months)