The paper presents a theoretical perspective based on the assumption that in education we deal with the “Problem of the invisible.” The modern medicine would have been impossible without the invention of microscope that rendered previously invisible class of organism visible. The authors’ contention is that in the educational research we are unable to detect a certain kind of phenomena, much less measure it: The relationships among the participants of the educational process. The structure of relationships may dramatically affect the impact of various educational interventions, which on average may produce low or contradictory effect sizes. Human relations may be the ultimate hidden variable of the education research.