TABLE 3.

Considerations for the ATE for exposures A₁, … , A₄; the same issues arise in estimation of the ATT and ATNT

Exposure	Estimand	Comments
A1	ITT effect	Randomization ensures unbiased estimation using simple contrasts
A3	ATE|A1 = 1, or ATE|A1 = 0	Effect of starting breastfeeding in a world where all (or no) women are offered the program. If we do not condition on A1, then we mix the two populations (or two “worlds”), which would never coexist outside of a trial where only half of women are offered the intervention. Furthermore, A2 is an effect modifier. Thus, correct specification of the outcome model requires an A2A3 term, and the ATE must then marginalize over the distribution of A2. Note that the conditioning on A1 is not relevant for estimating the causal effect of A2, as A1 has the role of an instrument for A2, but not for A3 or indeed for A4
A4	ATE|A3 = 1	There is no support in the data for an effect of A4 in women with A3 = 0. Note also that A4 = 0 is a mixture of durations of breastfeeding, potentially from 1 day up to just shy of 3 months. The consistency assumption implies that its estimated effect refers to settings with the same distribution of breastfeeding discontinuation times. An equivalent statement holds for the interpretation of A3 = 0 in the row above

Abbreviation: ITT = Intention‐to‐treat.