We have also demonstrated that the upper limit for power in an MR study is approximately the power for the reduced-form estimator, although this upper limit appears to be slightly higher for subsample IV estimators than for 2-sample IV estimators. This may be due to the slight residual bias in the direction of the observational estimate in the subsample case and in the direction of the null in the 2-sample case. Thus, in theory, exposure data are not needed for a fully powered test of the hypothesis that an exposure is causally related to an outcome if the reduced-form estimator is used (2). However, the reduced-form method does not produce a causal estimate for the effect of the exposure on the outcome and so does not allow the researcher to know whether a null finding is due to lack of a causal association or lack of power (for example, if the confidence interval for the IV estimate still includes the observational estimate).
