MR Dictionary

An MR analysis that tries to differentiate whether factor A is a cause or a consequence of factor B or whether there is a true bidirectional causal effect between A and B.

Genetic variants for ‘risk factor’ A and ‘outcome’ B need to fulfil the same assumptions as IVs. This requires genetic variants for both variables (A and B) to test each of the three possibilities: A causes B; B causes A; or there is a true bidirectional causal relationship between A and B. In comparing results in both directions, caution is required to take account of differences in statistical power related to the IV-risk factor association.

Bidirectional Mendelian randomization. Adapted from Richmond et al.
Figure 2.1 - Bidirectional Mendelian randomization. Adapted from Richmond et al.

References

Other terms in 'Definition of MR and study designs':