A car manufacturer created a new voice-control system that allows drivers to have limited use of their phones while keeping both hands on the wheel. They want to know if the system results in drivers making fewer mistakes, so they recruit volunteers for an experiment. Each subject is randomly assigned to one of three groups: voice-controlled phone use, regular phone use, or no phone use at all. Subjects drive the same simulated course while researchers send notifications at set moments to the drivers who can use phones. The researchers will then compare how many mistakes are made between the groups.
What is the primary purpose of including a group who doesn't use a phone at all?