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.
- Davey Smith G, Hemani G. Mendelian randomization: genetic anchors for causal inference in epidemiological studies. Hum Mol Genet 2014;23:R89-R98.
- Richmond RC, Davey Smith G, Ness AR, den Hoed M, McMahon G, Timpson NJ. Assessing causality in the association between child adiposity and physical activity levels: a Mendelian randomization analysis. PLoS Med 2014;11:e1001618.
Other terms in 'Definition of MR and study designs':
- Binary exposure MR
- Factorial MR
- Instrumental variable (IV)
- Mendelian randomization (MR)
- Multivariable MR
- One-sample MR
- Two-sample MR
- Two-sample MR with individual participant data (IPD)
- Two-sample MR with summary (aggregate) data
- Two-step/Mediation MR