We use past experiences every day when we choose one therapy over another; we frequently base our decisions on the relative probability that a particular treatment will be successful in an individual patient. Preoperative risk score systems are an essential tool for risk assessment in cardiac surgery. If we use just any risk stratification we will make diagnostic errors. During the last decade the examination of the performance of cardiac surgery risk stratification systems became very popular. There are a lot of studies, which show that risk stratification systems have high predictive value but they overpredict mortality rates for sample population. When reading these articles it is unclear what influences the mortality overprediction? We review main principles of receiver operating characteristic curve use for risk stratification systems' performance assessment and describe basic statistical explanations regarding errors in mortality prediction.