Question

When discussing validity threats, how does selection bias cause a problem? How does single group threat cause a problem?

Answer 1

Selection bias can arise in studies because groups of participants may differ in ways other than the interventions or exposures under investigation. When this is the case, the results of the study are biased by confounding.

There are various kinds of selection bias;

Statistical regression--It is also known as regression to the mean. This threat is caused by the selection of subjects on the basis of extreme scores or characteristics.

Selection of subjects--the biases which may result in selection of comparison groups. Randomization (Random assignment) of group membership is a counter-attack against this threat.

Selection-maturation interaction--the selection of comparison groups and maturation interacting which may lead to confounding outcomes, and erroneous interpretation that the treatment caused the effect.

Selection bias can have varying effects, and the magnitude of its impact and the direction of the effect is often hard to determine.

Single Group threat;

A group is introduced to a treatment or condition and then observed for changes which are attributed to the treatment. A single group is studied only once.

The Problems with Single Group design are:

• A total lack of control. Also, it is of very little scientific value as securing scientific evidence to make a comparison, and recording differences or contrasts.
• There is also a tendency to have the error of misplaced precision, where the researcher engages in tedious collection of specific detail, careful observation, testing and etc., and misinterprets this as obtaining good research. However you can not misinterpret that a detailed data collection procedure equals a good design.
• History, maturation, selection, mortality and interaction of selection and the experimental variable are all threats to the internal validity of Single Group design.