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===Experimental studies===
===Experimental studies===
In experimental studies (also known as '''intervention studies'''), the investigator allocates the exposure of interest to a selection of the participants prior to following them up. The allocation of the exposure should be randomised, and there should be a clear control group which does not receive the exposure (known as a '''Randomised Controlled Trial'''). Ideally, 'blinding' of participants and investigators to the treatment allocation should also be performed whenever possible in order to reduce any differences between the groups. As biases and confounding are reduced through the randomised allocation of exposure, these studies provide the best quality of evidence of any single study type. However, in a similar fashion to cohort studies, they can be very costly, both in terms of money and time. Additionally, in the case of suspected harmful exposures, randomised controlled trials may not be an ethical option, and so they are commonly used when investigating interventions which are suspected to be beneficial.