Thus, the potential gains in cost-efficiency we describe in this paper relate only to reducing the amount of exposure data that are needed. For most MR studies, genetic data and outcome data will be needed for very large numbers of participants to achieve adequate power (7), regardless of how much exposure data are generated. This is a major challenge for MR study design, especially considering that most genetic IVs are not especially strong. A possible solution to this is the use of multiple IVs, when available (7, 16).
