Selection bias is a term used in research that describes an unbalanced selection of study participants, which can cause the study's results to be invalid or not representative of the people it is supposed to describe. For instance, a study that uses undergraduate students under the age of 25 to describe how memory works for all people in the general population is an example of selection bias.
Mild traumatic brain injury has been a controversial research subject, primarily because symptoms that are thought to be related to the brain injury are also common to the general public. A recent study comparing the differences between a typical participant group for a mild traumatic brain injury study versus those that were screened for confounding symptoms found that only 3-4% of the typical participant group passed the stricter screening criteria.
Furthermore, study results were significantly different when using stricter screening criteria. Loss of consciousness, disorientation, and amnesia were more common in participants who passed the stricter screening. In addition, those who had passed the stricter screening were much more likely to have been in a car accident rather than a ground fall, whereas in typical participant groups, the two incident rates are more equal.
Luoto TM, Tenovuo O, Kataja A, et al. Who gets recruited in mild traumatic brain injury research? Journal of Neurotrauma. (September 2012).