MR Dictionary

MR for testing critical or sensitive periods

As genetic variants are fixed at conception, in general, they instrument a life-course predisposition to a particular risk factor. This makes MR less prone to regression dilution bias, but such genetic variants might be unsuited to testing the effects of risk factors that are believed to act during critical (a time period in which the risk factor is hypothesised to solely have its effect) or sensitive (a period during which the risk factor is hypothesised to have the greatest effect) periods. 

If the research question is about determining an effect in a critical or sensitive period (i.e., showing an exposure only affects an outcome when it occurs during a specific age range and not at other life times or has a stronger effect when it occurs during a specific time of life), MR may not be a useful method as the result it will provide is the mean effect of the exposure across all/most of the life course. As GWASs of trajectories (e.g., change in weight or height across infancy childhood and into early adulthood) are undertaken, they may provide genetic IVs that could be used in MR to compare change in an exposure between two ages with change in the same exposure between two later ages and compare these to see if there are differences in the effect at different ages. This is likely to need very large sample sizes. 


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