BookmarkSubscribeRSS Feed
Obsidian | Level 7



My question concerns statistical theory and not SAS per se. However, there are a lot of qualified statisticians on this forum, so I hope no one takes offence in me posting this here 😊


I am working with a clinical study that includes one interim analysis ¾-way into the trial in addition to the final analysis at the end. At the interim analysis (using data from 60 subjects), the primary endpoint variable was tested with a hypothesis test at a very low alpha of 0.1%, conserving the rest of the alpha (4.9%) to the final analysis (the O’brien-Fleming method). The study protocol stipulates that the study should be stopped in case the interim analysis is significant. Otherwise, the sample size should be re-estimated based on the interim analysis findings. The target number of subjects at the end of trial is currently 80, as specified in the protocol.


The result of the interim hypothesis test yielded a p-value of around 0.017. Hence, a very promising result, although, not significant at the predefined alpha for the interim analysis.


Can the study be stopped now? I assume no, since we did not meet the 0.1% alpha level in the interim analysis. However, clearly, if the study was stopped, significance would be attained, since there is 4.9% alpha remaining. Can there be grounds to let the study continue, but not all the way to 80 subjects? I.e. this would amount to a decrease in sample size as a result of the sample size re-estimation. However, it is not clear to me how that can be done. I could perform a conditional power calculation, but to my knowledge this only serves to answer the question of the original target sample size would be enough or if it needs to be increased. I assume I cannot use this framework to decrease the sample size?


Thanks in advance for any input.

Ready to join fellow brilliant minds for the SAS Hackathon?

Build your skills. Make connections. Enjoy creative freedom. Maybe change the world. Registration is now open through August 30th. Visit the SAS Hackathon homepage.

Register today!
What is ANOVA?

ANOVA, or Analysis Of Variance, is used to compare the averages or means of two or more populations to better understand how they differ. Watch this tutorial for more.

Find more tutorials on the SAS Users YouTube channel.

Discussion stats
  • 0 replies
  • 1 like
  • 1 in conversation