  The ICH E9(R1) addendum (2019) proposed principal stratification (PS) as one
of five strategies for dealing with intercurrent events. Therefore,
understanding the strengths, limitations, and assumptions of PS is important
for the broad community of clinical trialists. Many approaches have been
developed under the general framework of PS in different areas of research,
including experimental and observational studies. These diverse applications
have utilized a diverse set of tools and assumptions. Thus, need exists to
present these approaches in a unifying manner. The goal of this tutorial is
threefold. First, we provide a coherent and unifying description of PS. Second,
we emphasize that estimation of effects within PS relies on strong assumptions
and we thoroughly examine the consequences of these assumptions to understand
in which situations certain assumptions are reasonable. Finally, we provide an
overview of a variety of key methods for PS analysis and use a real clinical
trial example to illustrate them. Examples of code for implementation of some
of these approaches are given in supplemental materials.
