Generalizability vs. Transportability in Trials

I agree with your thoughts. Causal object explication clarifies “what exactly is the effect we are defining?” and external validity address a different problem, which is “what population is our causal object defined over?” As eligibility changes in a trial, these trials report different causal objects because the population component differs and that population specification is where external validity enters.

If our causal object is:

E[Y(1)−Y(0)|S=1]

then asking whether it generalizes to the broader population is asking about a different causal object:

E[Y(1)−Y(0)]

So external validity in analytical studies is essentially about applicability of one causal object to another population – and I propose we use the term applicability throughout – replacing transportability with causal applicability