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Background: The internet is increasingly being used to conduct randomized controlled trials (RCTs). Knowledge of the types of interventions evaluated and the methodological quality of these trials could inform decisions about whether to conduct future trials using conventional methods, fully online or a mixture of the two. Objective: To identify and describe the scope of internet-based RCTs for human health condition interventions and evaluate their methodological quality. Methods: A systematic review of RCTs of any health intervention conducted fully or primarily on the internet was carried out. Results: 23 fully and 27 primarily internet-based RCTs were identified. The first was conducted in 2000. The majority of trials evaluated interventions that involved providing health information to participants, but a few evaluated self-administered interventions (eg, valerian, stretching). Methodological quality was variable and the methods were generally poorly reported. The risk of bias was low in only a small number of trials; most had substantial methodological shortcomings. Only one trial was identified as meeting all criteria for adequate methodological quality. A particular problem was high rates of loss to follow-up (fully online: mean 47%; primarily online: mean 36%). Conclusions: It is theoretically possible but perhaps difficult to test the effectiveness of health interventions rigorously with RCTs conducted fully or primarily over the internet. The use of the internet to conduct trials is more suited to pragmatic rather than explanatory trials. The main limitation of these trials is that they typically experience high rates of loss to follow-up. Methodological standards now accepted for traditional RCTs needs to be evident for online RCTs as well, especially in reporting of their methods.

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