MR Dictionary

Independence assumption

Synonyms: IV2 assumption, second MR assumption

Known as the second IV assumption (and sometimes also referred to as the “exchangeability” assumption) this states that there are no confounders of the association between the IVs and the outcome.

As genetic variants are determined at conception it is not possible for them to be affected by confounders of risk factor-outcome associations. When referring to the second IV assumption, factors that could influence the genetic variants and outcome include population stratification, dynastic effects and assortative mating. Generally, genetic variants can influence confounders of the risk factor-outcome association and would be analagous to horizontal pleiotropy. If they do and these confounders are not measured / not controlled for in the MR analysis, then a path from the genetic IV via these confounders could bias the causal effect estimate. As noted under "confounding" in both one-sample and two-sample MR, associations of the genetic IV with potential confounders should be undertaken and methods such as multivariable MR used to control for these where possible. 

References

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