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.
Error due to systematic differences in characteristics between those who take part in a study and those who do not. Selection bias affects conclusions and generalizations that are made regarding the study sample.
Bias that is introduced into a study when study participants (subjects) are not representative of the population to which inferences are to be made. See Selection.
Distortion or falsification of study results (bias) that can occur when study subjects are assigned to groups.
The selection process did not yeild a sample that is representative of the population demographically
An error in the estimates of disease prevalence and other population parameters that results when the criteria for admission to a study produces systematic differences between the study population and the clinically relevant population.
A bias in assignment or a confounding variable that arises from study design rather than by chance. These can occur when the study and control groups are chosen so that they differ from each other by one or more factors that might affect the outcome of the study.
Statistical errors due to systematic differences in characteristics between those selected for a study and those who are not.
A bias in the way the experimental and control or comparison groups are selected, resulting in pre-existing differences between the groups that may serve as confounding factors.
Systematic error in sampling the population.
Systematic error in creating intervention groups, causing them to differ with respect to prognosis. That is, the groups differ in measured or unmeasured baseline characteristics because of the way in which participants were selected for the study or assigned to their study groups. The term is also used to mean that the participants are not representative of the population of all possible participants. See also Allocation concealment, External validity.
An error in choosing the individuals or groups to take part in a study. Ideally, the subjects in a study should be very similar to one another and to the larger population from which they are drawn (for example, all individuals with the same disease or condition). If there are important differences, the results of the study may not be valid.
Selection bias, sometimes referred to as the selection effect, is the error of distorting a statistical analysis due to the methodology of how the samples are collected. For example the sample selection may involve pre- or post-selecting the samples that may preferentially include or exclude certain kinds of results. Typically this causes measures of statistical significance to appear much stronger than they are, but it is also possible to cause completely illusory artifacts.