Experiments, including clinical trials, differ considerably in the methods used to assign participants to study conditions (or study arms) and to deliver interventions to those participants. This website provides information related to the design and analysis of experiments in which 1) participants are assigned in groups (or clusters) OR participants are assigned individually but receive at least some of their intervention with other participants or through a change agent shared with other participants, and 2) individual observations are analyzed to evaluate the effect of the intervention.
These studies may differ in their units of assignment (groups or clusters vs individuals), but they face the same expectation of positive correlation among observations taken on members who receive their intervention together, whether in pre-existing groups or in groups created by the investigator, and whether in person or through a common change agent.
The material is relevant for both human and animal studies and for both basic and applied research. And while it is important for investigators to become familiar with the issues presented on this website, it is even more important that they collaborate with a methodologist who is familiar with these issues.
In a group-randomized trial, also called a cluster-randomized trial, groups or clusters are randomized to study conditions, and observations are taken on the members of those groups.6, 17, 19, 23, 38 This design is common in public health, where the units of assignment may be schools, worksites, clinics, or whole communities, and the units of observation are the students, employees, patients, or residents within those groups. It is common in animal research, where the units of assignment may be litters of mice or rats and the units of observation are individual animals. It is also common in clinical research, where the units of assignment may be patients and the units of observation are individual teeth or eyes. Special methods are required for analysis and sample size estimation for these studies and investigators will need to show in their application that their methods are appropriate.
Learn more about the design and analysis of group- or cluster-randomized trials.
In an individually randomized group-treatment trial, also called a partially clustered design, individuals are randomized to study conditions but receive at least some of their intervention with other participants or through a change agent shared with other participants.4, 5, 33-35, 45-47, 50, 51 This design is common in public health, where participants randomized to a weight-loss intervention may attend a class where they receive some of their intervention with other participants.31 It is common in psychology, where a therapist may treat multiple participants, either as a group or as individuals.3, 11, 53 Special methods are required for analysis and sample size estimation for these studies, and investigators will need to show that their methods are appropriate.
Learn more about the design and analysis of individually randomized group-treatment trials or partially clustered designs.
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