When researchers inadvertently create a flaw in their experiment that is a threat to internal validity, it is known as a design confound. If these flaws coincide with the experimental manipulation and call into question whether or not the manipulation or the flaws affected the dependent variable, this is known as systematic variability. If the flawed part of the experiment does not coincide with a specific group, and is introduced at random, this is known as __ and does not necessarily pose a threat to internal validity.

1. selection effect
2. random sampling
3. unsystematic variability
4. none of the above