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September 2nd, 2017, 12:43 PM  #1 
Newbie Joined: Sep 2017 From: Oxford Posts: 1 Thanks: 0  Probability re Placebo
Any help appreciated to calculate the below. A clinical trial has two arms. Arm 1 is placebo. The results generated were 17 responders out of 44 patients (38.6%) Arm 2 is the active trial. The results generated were 49 responders out of 90 patients (54.4%) How do I calculate the p number to show the likelihood of the active arm obtaining that statistical result by chance, i.e. the chance of there being no real difference between the active arm and placebo? Anything under p=0.05 will be statistically significant, which is what I am trying to assess. Thanks in advance Last edited by siciliankan; September 2nd, 2017 at 12:44 PM. Reason: Extra information 
September 10th, 2017, 04:17 AM  #2 
Member Joined: May 2013 Posts: 34 Thanks: 1 
so lets create the probability square. Code: prob(react to placebo)  prob(not react to placebo)  prob(react to actual)  49/90  prob(not react to actual)  41/90  17/44  27/44  1 
October 27th, 2017, 11:25 AM  #3 
Newbie Joined: Oct 2017 From: US Posts: 13 Thanks: 1 
Just put the following numbers into this chisquare calculator 17 27 49 41 you will get a p value of 0.085631065798, which is larger than 0.05. Therefore, it is not significant at 0.05 level and you can not reject the null hypothesis of independence that placebo and trial are the same in 'react or not'. Last edited by James White; October 27th, 2017 at 11:43 AM. 

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