Table 1.

Summary table of features of instrumental variable methods considered in this paper.

Ratio method	Can only be used with a single IV. Can be used with summarized data.
Two-stage methods	Efficient, but biased with weak instruments. Robust to misspecification in the first-stage model. Flexible to incorporate different exposure–outcome models.
Likelihood-based methods	Asymptotically efficient and less biased provided that the parametric assumptions hold. Requires coding by hand outside of the linear case.
Semi-parametric methods	Less efficient, less stringent assumptions. No guarantee of unique estimate in finite samples.