MR Dictionary

Weak instrument bias

Instrument strength is determined by the magnitude and precision of association of the genetic IVs with the risk factor of interest. 

The R2 and F-statistic for the association of the genetic IV with the risk factor provide an indication of instrument strength (the higher the value of both, the stronger the instrument and the greater statistical power in MR analyses). For reference, genetic IVs were traditionally considered to be sufficient if the corresponding F-statistic was >10. In one-sample MR, weak instruments tend to bias towards the confounded risk factor-outcome association; whereas, in two-sample MR (with non-overlapping samples) the bias is expected to be towards the null. 

References

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