In any sort of scientific test, it's important is to control the conditions to make sure that any variables are neutralized, and that the experiments can be repeated with consistent results. An experiment that gives the same data over and over again can usually be seen as an accurate way of obtaining information on a subject.
As a means of reducing human error, Mercedes-Benz is utilizing automated drivers, using special "robots" that are capable of steering, braking and using the throttle (seen above). Unlike Volkswagen, the robots cannot drive themselves "autonomously", but rather they follow a specific pattern to drive around a closed course. Mercedes-Benz engineers have an override switch to shut down the cars as well.
The robots allow Mercedes-Benz engineers to test the cars using drivers that have better reaction times and are more consistent than human drivers. This in turn lets the engineers get better data to use when developing the cars, as well as information on how they should behave in potentially dangerous situations without putting human life at risk.
No word on the cost of these systems, but you can bet that it's more than your house.
More: Mercedes-Benz Automated Driving Helps Crash Testing By Improving Data