Discussion of 'Estimating time-varying causal excursion effect in mobile health with binary outcomes' by T. Qian et al

TL;DR

This paper discusses the 'excursion effect' methodology in mobile health, clarifying its relationship to existing models and exploring its dependence on study design and potential for treatment protocol modifications.

Contribution

It provides a detailed analysis of the excursion effect, relating it to prior models and clarifying methodological differences and implications.

Findings

Excursion effect relates closely to existing marginal structural models.

Methodological differences impact the interpretation of the excursion effect.

The effect's dependence on study design influences its application for treatment modifications.

Abstract

We discuss the recent paper on "excursion effect" by T. Qian et al. (2020). We show that the methods presented have close relationships to others in the literature, in particular to a series of papers by Robins, Hern\'{a}n and collaborators on analyzing observational studies as a series of randomized trials. There is also a close relationship to the history-restricted and the history-adjusted marginal structural models (MSM). Important differences and their methodological implications are clarified. We also demonstrate that the excursion effect can depend on the design and discuss its suitability for modifying the treatment protocol.