  Just-in-time adaptive interventions (JITAIs) are time-varying adaptive
interventions that use frequent opportunities for the intervention to be
adapted such as weekly, daily, or even many times a day. This high intensity of
adaptation is facilitated by the ability of digital technology to continuously
collect information about an individual's current context and deliver
treatments adapted to this information. The micro-randomized trial (MRT) has
emerged for use in informing the construction of JITAIs. MRTs operate in, and
take advantage of, the rapidly time-varying digital intervention environment.
MRTs can be used to address research questions about whether and under what
circumstances particular components of a JITAI are effective, with the ultimate
objective of developing effective and efficient components. The purpose of this
article is to clarify why, when, and how to use MRTs; to highlight elements
that must be considered when designing and implementing an MRT; and to discuss
the possibilities this emerging optimization trial design offers for future
research in the behavioral sciences, education, and other fields. We briefly
review key elements of JITAIs, and then describe three case studies of MRTs,
each of which highlights research questions that can be addressed using the MRT
and experimental design considerations that might arise. We also discuss a
variety of considerations that go into planning and designing an MRT, using the
case studies as examples.
