What is selection bias?
Selection bias occurs when those enrolling patients are aware of the group to which the next enrolled patient will be allocated, either because of a non-random sequence that is predictable, or because a truly random sequence was not concealed from the researcher. This may lead to systematic selection bias, where the researcher chooses not to recruit a participant into that specific group because of knowledge of that participant s likely prognostic characteristics, and a desire for one group to do better than the other.