# C-index (concordance index), Harrell's and Uno's

Source: https://onco.cc/terms/concordance-index/  
OnCo record `concordance-index` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

The C-index is the fraction of patient pairs in which the model ranked the patient who had the event sooner as higher risk; 0.5 is a coin toss and 1.0 is perfect ranking.

## Summary

Harrell and colleagues introduced the concordance index as the probability that, for a random pair of patients, the one predicted to be at higher risk has the earlier event; Uno and colleagues gave a version that weights pairs by the inverse probability of censoring so that heavy censoring does not bias it upwards. A pooled pan-cancer C-index is inflated by tissue of origin (leukaemia patients die sooner than thyroid patients), so within-type or stratified C-indices are the honest measure. The C-index ranks; it says nothing about calibration.

## Fields

- Kind: Term
- Last checked: 2026-09-24
- Also known as: C-index; concordance index; Harrell's C-index; Harrell's C; Uno's C-index; Uno's C; C-statistic; pooled C-index; stratified C-index; within-type C-index
- Tags: cansim-terms

## Notes

- Listed in the CanSim terms map 1.0.0 (docs/onco/terms.json, generated 2026-09-24), CC BY 4.0, attribution: CanSim project, an open, public-data-first cancer foundation-model programme; CanSim page path /terms/c-index.

## Sources

- Harrell et al., Evaluating the yield of medical tests (JAMA 1982): https://doi.org/10.1001/jama.1982.03320430047030
- Uno et al., On the C-statistics for evaluating overall adequacy of risk prediction procedures with censored survival data (Statistics in Medicine 2011): https://doi.org/10.1002/sim.4154

## Connected records

- terms: [Calibration: reliability diagrams and the Brier score](https://onco.cc/terms/calibration/), [Cancer AI vocabulary (CanSim terms map)](https://onco.cc/terms/cancer-ai-vocabulary/), [Censoring and events in survival data](https://onco.cc/terms/censoring-and-events/), [Hazard ratio (HR)](https://onco.cc/terms/hazard-ratio/), [Risk score and risk percentile](https://onco.cc/terms/prognosis-risk-percentile/), [ROC-AUC, PR-AUC and time-dependent AUC](https://onco.cc/terms/roc-auc/), [Tissue-of-origin signal in tumour data](https://onco.cc/terms/organ-of-origin-signal/), [Univariate Cox scores and time-dependent metrics](https://onco.cc/terms/time-dependent-auc/)

---
JSON: https://onco.cc/api/v1/entities/concordance-index.json