Unit of randomization individuals or groups

All the discussions above have assumed that an individual patient will be the unit of randomization, and for most cancer treatment trials this is certainly the most appropriate approach. In other settings, particularly primary care and hence in cancer screening and prevention trials, an alternative design has sometimes been used in which, rather than randomizing individual patients, groups of clearly defined patients are randomized. For example, one might randomize primary care practices, such that every patient within a given practice receives the same treatment. This is known as a cluster- (or group-) randomized trial. While there are situations in which this might be appropriate, it is far from problem-free. The use of this design in many of the trials of screening for breast cancer, and the consequent difficulties in interpretation of the results has contributed to the confusion over the impact of breast screening on breast cancer mortality.

It is worth emphasizing that the 'randomization units', 'experimental units' and 'observation units' may be quite different in a cluster randomized trial compared with a conventional trial. In a conventional individual patient-randomized trial, the randomization unit, experimental unit and observation unit are all the individual patient, that is, the patient is randomized, treated individually and their individual outcome data contribute to the overall assessment of treatment. With cluster randomized trials, the randomization unit is a defined group, but the 'experimental' unit may be an individual - for example the primary care physician responsible for that group of patients might be randomized to receive or not to receive an education package. The observation unit maybe the group as a whole, or the individual patients. Even if individual patient outcome data are used, the effect of the original cluster-randomized design cannot be ignored in the analysis.

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