title: Sample size calculations for micro-randomized trials in mHealth.
abstract: The use and development of mobile interventions are experiencing rapid growth. In "just-in-time" mobile interventions, treatments are provided via a mobile device, and they are intended to help an individual make healthy decisions 'in the moment,' and thus have a proximal, near future impact. Currently, the development of mobile interventions is proceeding at a much faster pace than that of associated data science methods. A first step toward developing data-based methods is to provide an experimental design for testing the proximal effects of these just-in-time treatments. In this paper, we propose a 'micro-randomized' trial design for this purpose. In a micro-randomized trial, treatments are sequentially randomized throughout the conduct of the study, with the result that each participant may be randomized at the 100s or 1000s of occasions at which a treatment might be provided. Further, we develop a test statistic for assessing the proximal effect of a treatment as well as an associated sample size calculator. We conduct simulation evaluations of the sample size calculator in various settings. Rules of thumb that might be used in designing a micro-randomized trial are discussed. This work is motivated by our collaboration on the HeartSteps mobile application designed to increase physical activity. Copyright © 2015 John Wiley & Sons, Ltd.
authors.affiliation: Department of Statistics, University of Michigan, MI, 48109, Ann Arbor, U.S.A.
authors.forename: Peng
authors.initials: P
authors.lastname: Liao
authors.affiliation: School of Information, University of Michigan, MI, 48109, Ann Arbor, U.S.A.
authors.forename: Predrag
authors.initials: P
authors.lastname: Klasnja
authors.affiliation: Department of Statistics, University of Michigan, MI, 48109, Ann Arbor, U.S.A.
authors.forename: Ambuj
authors.initials: A
authors.lastname: Tewari
authors.affiliation: Department of Statistics, University of Michigan, MI, 48109, Ann Arbor, U.S.A.
authors.forename: Susan A
authors.initials: SA
authors.lastname: Murphy
mesh_terms: D015444:Exercise; D006293:Health Promotion / Q000379:methods; D006801:Humans; D063731:Mobile Applications / Q000706:statistics & numerical data; D016032:Randomized Controlled Trials as Topic / Q000379:methods / Q000592:standards* / Q000706:statistics & numerical data; D018401:Sample Size; D013223:Statistics as Topic
keywords: health; mirco-randomized trial; sample size calculation