Weak Instrument Bias

Weak instrument bias occurs when the chosen instrument has a very low correlation with the endogenous variable it is meant to explain. This leads to unstable and unreliable estimates of the causal effect, even with large sample sizes.

In financial modeling, finding strong, valid instruments is notoriously difficult, making this a common challenge. If the instrument is weak, the standard errors become inflated, and the resulting coefficients may be far from the true causal value.

This bias can lead to incorrect conclusions about the effectiveness of market interventions or the drivers of volatility. Researchers must carefully test the strength of their instruments before proceeding with 2SLS or other causal models.

It highlights the importance of theoretical grounding and data quality in empirical finance. A weak instrument is often worse than no instrument at all, as it provides a false sense of precision.