Trials (experimental studies)
Trials are commonly used for evaluating the effectiveness of interventions, especially drug interventions. Most commonly, a new drug is compared with existing drugs or a placebo. The trials can be categorised as randomised or non-randomised depending on the method of allocating people to receiving either the interventional drug or placebo/existing drug.
In a randomised trial, the allocation of participants into either group is ‘random’, that is, every participant who enters the study has an equal chance of receiving either the intervention or placebo. In non-randomised trials, the process of allocation is not random and may be predictable.
Trials (whether randomised or non-randomised) are essentially similar to cohort studies (see Chapter 10). The key difference is that participants are allocated to either an intervention group or a placebo group by the investigators. Figure 11a illustrates this whereby the participants are randomly allocated to an intervention group and a control group. The groups are then followed up to assess the development of the desired outcome in each group. A variation of this design is to have a period of ‘wash-out’ whereby any residual effects of the treatments are eliminated followed by the two groups then crossing over to the other arm. The groups are then followed up for assessment of the outcome (crossover trial design).