c-index

The c-index, or concordance index, is a measure used to evaluate how well a model predicts outcomes, especially in survival analysis. It reflects the probability that, for a randomly chosen pair of individuals, the one who experiences an event (like failure or death) earlier is also predicted by the model to be at higher risk. A c-index of 0.5 indicates the model is no better than random guessing, while a value of 1.0 means perfect prediction. Essentially, it's a way to assess the model’s accuracy in ranking individuals according to their risk.