[`ASSISTant`](https://cran.r-project.org/package=ASSISTant) is an R package for a novel group-sequential adaptive trial. The design is motivated by a randomized controlled trial to compare an endovascular procedure with conventional medical treatment for stroke patients; see Lai, Lavori and Liao [-@Lai2014191]. The endovascular procedure may be effective only in a subgroup of patients not known at the design stage but may be learned statistically from the data collected during the course of the trial. The group-sequential design implemented in `ASSISTant` incorporates adaptive choice of the patient subgroup among several possibilities which includes the entire patient population as a choice. Appropriate Type I and type II errors of a test can be defined in this setting and the design maintains a prescribed type I error by using the closed testing principle in multiple testing.
This adaptive design underlies the [NIH DIFFUSE-3](https://www.nihstrokenet.org/clinical-trials/acute-interventional-trials/defuse-3) trial currently underway. The package is on CRAN [-@assistant] and github.
