He calls in a group for an experiment, but in reality there’s only one subject in the group – everyone else knows what is going on, and is part of it.
The group is shown two images:
The group is asked which line, A B C, is the same length as the line on the left. They are seated in a circle so they answer one by one, and the subject always answers last.
The answer appears to be C, right? There were multiple trials where everyone would give the same answers, then some would answer wrong, then some would answer right.
The goal is to see if the subject will “conform” to the group and answer what everyone else has answered or if the subject won’t conform and answer what he/she thinks is the correct answer.