CT-AI: Certified Tester AI Testing Version 2.0

Consider a machine-learning model that attempts to predict whether a patient is at risk of stroke. The model gathers information on each patient about blood pressure, red blood cell count, smoking status, history of heart disease, cholesterol level, and demographics. It then uses a decision tree to predict whether the associated patient is likely to have a stroke in the near future. After the model is created using a training dataset, it is used to predict stroke risk for 80 additional patients. The table below shows a confusion matrix indicating whether the model made correct or incorrect predictions.

The testers calculated what they believe is an appropriate functional performance metric for the model, obtaining a value of 2/3, or 0.6667.

Which metric did the testers calculate?