Threats to Internal Validity
5. Statistical regression - an effect caused by a tendency for subjects to 'regress' from extreme high or low initial scores back to a more 'moderate' or 'average' level of performance on subsequent tests.
6. Differential selection of subjects - an effect due to the groups of subjects not being randomly assigned or selected; but rather, a selection factor is operating such that the groups are not equivalent.
5. In a study involving reading instruction, subjects initially grouped on the factor of 'poor' pretest reading scores show considerably greater gains than the average readers. It is important to keep in mind that this is a quantitative artifact that is bound to happen anyway, regardless of the quality or impact of the 'treatment' (e.g., reading instruction), simply due to the 'poor' initial grouping. 6. The 'experimental' group in an instructional experiment happens to consist of a 'high-IQ' class, while the control group happens to be an 'average-IQ' class. The question arises: wouldn't the first group have been likely to do better anyway? regardless of the impact or quality of the 'treatment?'