In most educational research, a pretest is given, followed by treatment and post-test. Reliability of instrument, observational methods are quite susceptible to change. In experiments where there is two or more groups, there can be systematic differences between the groups that can account for the effect. Really apparent in longitudinal studies. Treatment Diffusion Treatment effect of one group affecting other groups, especially if the treatment condition is perceived as highly desirable.
Proximity is key.
Two different teaching methods and the teachers talk to each other, share ideas, etc. 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.