Editing Internal validity (section)

===Selection bias===
Selection bias refers to the problem that, at pre-test, differences between groups exist that may interact with the independent variable and thus be 'responsible' for the observed outcome. Researchers and participants bring to the experiment a myriad of characteristics, some learned and others inherent. For example, sex, weight, hair, eye, and skin color, personality, mental capabilities, and physical abilities, but also attitudes like motivation or willingness to participate.

During the selection step of the research study, if an unequal number of test subjects have similar subject-related variables there is a threat to the internal validity. For example, a researcher created two test groups, the experimental and the control groups.  The subjects in both groups are not alike with regard to the independent variable but similar in one or more of the subject-related variables.

Self-selection also has a negative effect on the interpretive power of the dependent variable.  This occurs often in online surveys where individuals of specific demographics opt into the test at higher rates than other demographics.