Skip to Main Content

We have a new app!

Take the Access library with you wherever you go—easy access to books, videos, images, podcasts, personalized features, and more.

Download the Access App here: iOS and Android. Learn more here!



  • imageWhen research involves time-related variables, such as survival and recurrence, we generally do not know the outcome for all patients at the time the study is published, so these outcomes are called censored.

  • imageObservations are doubly censored when not all patients enter the study at the same time.

  • imageAn example of why special methods are needed to analyze survival data helps illustrate the logic behind them.

  • imageLife table or actuarial methods were developed to show survival curves; although generally surpassed by Kaplan–Meier curves, they occasionally appear in the literature.

  • imageSurvival analysis gives patients credit for how long they have been in the study, even if the outcome has not yet occurred.

  • imageThe Kaplan–Meier procedure is the most commonly used method to illustrate survival curves.

  • imageEstimates of survival are less precise as the time from entry into the study becomes longer, because the number of patients in the study decreases.

  • imageSurvival curves can also be used to compare survival in two or more groups.

  • imageThe logrank statistic is one of the most commonly used methods to learn if two curves are significantly different.

  • imageThe hazard ratio is similar to the odds ratio; the difference is that the hazard ratio compares risk over time, while the odds ratio examines risk at a given time.

  • imageThe Mantel–Haenszel statistic is also used to compare curves, not just survival curves.

  • imageSeveral versions of the logrank statistic exist. The logrank statistic assumes that the risk of the outcome is the constant over time.

  • imageThe Mantel–Haenszel statistic essentially combines a number of 2 × 2 tables for an overall measure of difference.

  • imageThe hazard function gives the probability that an outcome will occur in a given period, assuming that the outcome has not occurred during previous periods.

  • imageThe intention-to-treat principle states that subjects are analyzed in the group to which they were assigned. It minimizes bias when there are treatment crossovers or dropouts.


Presenting Problem

Metastatic colorectal cancer may be treated by regorafenib alone or regorafenib in combination with chemotherapy. Lin et al (2018) studied the survival curves for patients on each treatment. They analyzed data from 61 patients that completed the study: 34 on regorafenib combined with chemotherapy and 27 received regorafenib alone.

The full article may be accessed here:


Many studies in medicine are designed to determine whether a new medication, a new treatment, or a new procedure will perform better than the one in current use. Although measures of short-term effects are of interest with efforts to provide more ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.