These are also (and more descriptively) known as Standardized Patients ("SPs"), because the main goal is to test the medical students' knowledge under standard conditions to enable examiners to compare scores, and to define acceptable performace for all students.
Sometimes, the students try to outwit the tests. In one recent test, the female SP presented with simulated bruises and exhibited many signs of physical abuse. Most students competently addressed the physical wounds, but did not address the fact of the abuse. They received low marks, and word spread about this test.
Later, the test was given to a different set of students. These students, forwarned by their peers, competently addressed the abuse...but forgot to address the injuries!