What characterizes a randomized controlled trial?

A randomized controlled trial (RCT) is primarily characterized by the use of chance to assign participants to different treatment groups. This random assignment helps ensure that the groups are comparable at the outset and minimizes biases that could affect the results. By using a method of chance, such as a random number generator or a coin flip, the researchers can avoid subjective influences, leading to more reliable data regarding the effectiveness of the interventions being tested.

In the context of RCTs, selected assignment or observation without intervention does not align with the fundamental principles of this type of research. Selected assignment might introduce bias into the treatment groups, while observation without intervention is more characteristic of observational studies rather than a controlled trial. Single-group assessments do not provide the necessary comparison that a randomized controlled trial aims to achieve; RCTs typically involve at least two groups: one receiving the treatment and another serving as a control. Therefore, the correct characterization of a randomized controlled trial is indeed that assignment is determined by chance, making B the most accurate choice.