Community Trial

Community trials, also called community intervention studies, are (mostly preventive) experimental studies with whole communities (such as cities or states) as experimental units; that is, interventions are assigned to all members in each of a number of communities. These are to be distinguished from field trials where interventions are assigned to healthy individuals in a community and from clinical trials in which interventions are assigned to patients in a clinical setting. Except for the experimental unit, the conduct of controlled community trials follow the same procedures as controlled clinical trials, including the requirement of informed consent of the communities meeting the eligibility criteria (e.g., consent being given by city mayors or state governors), randomization to treatment and control groups, and follow-up and measurement of endpoints. In contrast to clinical trials, blinding and double blinding are not generally used in community trials. Community trials are needed to evaluate directly the potential efficacy of large-scale public health interventions, and they are indispensable for the evaluation of interventions that cannot be allocated to individuals as experimental units. For example, in studying the effects on dental caries of adding fluoride to the water supply (see Example 2 below), Ast and colleagues found it impossible to add fluoride to drinking water for selected individuals; instead, a controlled community trial was conducted in which whole towns were allocated to receive fluoride in their water or not. Just as with other randomized controlled clinical trials, a randomized controlled community trial requires a sufficient number of communities to involve in the experiment for the randomization to achieve its purpose of reducing confounding bias. In reality, considerably fewer study units are capable of being randomized in community trials than in clinical trials, simply because the study units for the former are considerably larger than those for the latter. For this reason, community trials are often quasi-experimental studies rather than experimental studies and so are considered to have lower validity than clinical trials. However, community trials still have higher validity than all observational analytic and descriptive studies. After enough information has been accumulated from descriptive or observational studies about the risk factors and their potential for modification, community trials may then be conducted to assess the benefit of new public health programs. Below are two real examples of community trials, only the first example being a bona fide community trial.