Flowers of the First Law

Continuing our examination of “the flowers of the First Law” (http://www.mii.ucla.edu/causality/?p=1354), this posting looks at one of the most crucial questions in causal inference: “How generalizable are our randomized clinical trials?” Readers of this blog would be delighted to learn that one of our flowers provides an elegant and rather general answer to this question. I will describe this answer in the context of transportability theory (http://ftp.cs.ucla.edu/pub/stat ser/r400reprint.pdf), and compare it to the way researchers have attempted to tackle the problem using the language of ignorability. We will see that ignorability-type assumptions are fairly limited, both in their ability to define conditions that permit generalizations, and in our ability to justify them in specific applications.