Assignment bias is a term used in used in the analysis of research data for factors that can skew the results of a study. For instance, a research study compares test results from students at two different schools. Even if the researcher controls the age, gender and grade level of the students being studied they might not be able to control factors such as the ethnic background, school quality, family background, etc. of the students. This lack of control can adversely affect the reliability of the experiment.