The characteristics or expectations of experimenter can influence the results. Age, gender, race, can all create a bias or hostile environment. Observers and interviewers should be unaware of study. Must create guidelines to reassure that bias has not influenced the results. As people participate in studies, their awareness of participation can change behavior.
Subjects change behavior in response to experimental situation.
Act more favorably. Want to appear intelligent, competent, stable. Participation can increase because of novelty or diminished because they have found out they were not selected. Some argue, most notably Richard Snow, that educational research is too artificial and lacks generalizability. This reflects both real-life environments and learning characteristics of students.