Does anyone know what the specific metric is that they are using to compare Wat son to doctors? What does it mean that doctors make the correct treatment decisi

Does anyone know what the specific metric is that they are using to compare Wat son to doctors? What does it mean that doctors make the correct treatment decision 50% of the time? I don't doubt that Watson is better at this task, but it's difficult to have an appreciation of its ability without understanding the test.