MR Dictionary

Weak instrument bias

Weak instrument bias is the bias that occurs when the instrumental variable (IV) being used in an MR analysis is only weakly associated with the exposure.

The strength of IV in MR analyses is determined by the magnitude and precision of association of the genetic IVs with the exposure of interest. The R-squared and F-statistic for the association of the genetic IV with the exposure provide an indication of the strength of the IV or "instrument strength" (i.e., the higher the value of both, the stronger the IVand the greater statistical power in MR analyses). Genetic IVs were traditionally considered to be of sufficient power if the corresponding F-statistic was >10; however, this is not a rigid rule and an F-statistic <10 does not indicate that this IV should not be used, rather that weak instrument bias should be considered as an issue in analysis. In one-sample MR, weak IVs tend to bias towards the confounded association between the exposure and outcome; whereas, in two-sample MR (with non-overlapping samples) the bias is expected to be towards the null. 


Other terms in 'Sources of bias and limitations in MR':