  The micro-randomized trial (MRT) is a new experimental design which allows
for the investigation of the proximal effects of a "just-in-time" treatment,
often provided via a mobile device as part of a mobile health intervention. As
with a traditional randomized controlled trial, computing the minimum required
sample size to achieve a desired power is a crucial step in designing an MRT.
We present MRT-SS Calculator, an online sample-size calculator for
micro-randomized trials, built with R Shiny. MRT-SS Calculator requires
specification of time-varying patterns for the proximal treatment effect and
expected treatment availability. We illustrate the implementation of MRT-SS
Calculator using a mobile health trial, HeartSteps. The application can be
accessed from https://pengliao.shinyapps.io/mrt-calculator.
