Table 1.

Three key assumptions that must hold for a Mendelian randomisation study to be valid

Assumption	Description	Tools to assess plausibility
		Single sample	Two sample
Relevance assumption	The genetic variants associate with the risk factor of interest	The partial F statistic and partial r squared, or risk difference	Variants are associated with the risk factor in a large genome-wide study
Independence assumption	There are no unmeasured confounders of the associations between genetic variants and outcome	Covariate balance tests and bias component plots. Adjusting for principal components of population stratification	Evidence from large genome-wide association studies on the association of the genetic variants used as instruments with other baseline covariates
Exclusion restriction	The genetic variants affect the outcome only through their effect on the risk factor of interest	Biological knowledge, tests of association of the genetic variants and potential alternative mediating pathways	Evidence from large genome-wide association studies that the genetic variants associate with alternative pathways. MR Egger test for pleiotropy, Cook’s distance evaluation of outliers