Part 1 provides an overview of three kinds of randomized trials and their distinguishing characteristics: randomized clinical trials, individually randomized group-treatment (IRGT) trials, and group-randomized trials (GRTs) including both parallel GRTs and stepped wedge group-randomized trials (SWGRTs). 

Part 1 considers the risks of confounding in these designs and the main types of parallel GRTs and IRGT trials. It also reviews the nesting and membership structures in IRGT trials and the key features of SWGRTs. Part 1 closes with guidance on how to select among these four designs and a review of a priori matching, a priori stratification, and constrained randomization in these designs.