Potential biases introduced into a study by the selection of different types of people into treatment and comparison groups. As a result, the outcome differences may potentially be explained as a result of pre-existing differences between the groups, as opposed to the treatment itself.
Error due to systematic differences in characteristics between those who are selected for study and those who are not.
Could not the differences between the control group and the programme group be due to initial differences in their characteristics rather than the effects of the intervention we are trying to evaluate? See also control group, counterfactual situation, evaluation design, internal validity, programme group.