In a famous research study conducted between $1927$ and $1932$ at an electricalequipment manufacturing plant, experimenters measured the influence of a number of variables $($brightness of lights, room temperature, group pressure, working hours, and managerial leadership$)$ on the productivity of the employees.

The mojor finding of the study was that no matter what experimental treatment was employed, the production of the workers seemed to improve it scemed as though just knowing that they were being studied had a strong positive influence on them

An important statistical effect was named for this manufacturing plant. What is it$?$