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#1




Researchers conclusion was no drug effect!
A 1year study of a new drug to treat hypertension is conducted. One hundred patients with hypertension are enrolled; 50 patients are given the new drug and another 50 patients are given hydrochlorothiazide. All patients completed the trial. One noted unexpected effect is increased growth of scalp hair which occurred in those taking the new drug, a nonstatistically significant difference (p>0.10). This effect has also been reported in studies of other similar drugs in the new therapeutic class. The investigators of the study concluded that the new drug did not cause hair growth. Which of the following features of this study is most likely to affect the validity of this conclusion?
A) Differential followup B) Lead time bias C) Length of the study D) Sample size E) Selfselection 
#2




In this case 2 answers are plausible...
1. Differential followup 2. Sample Size But here they say that were NO drop out...so the answer should be Sample Size 
#3




bebix what about c...
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#4




Differential follow up aka attrition bias is a very common bias in cohort studies where cases drop out while in the study. You can call it a sub type of selection bias. In the given context it can surely thawrt validity but more so for results on blood pressures. Here. Issue is on hair growth. So i think it could just be sample size, Best study designs to find out side effects of any drug are special interventional studies that are done on large sample sizes probably in hundreds. Small sample cohort studies are not adequate for side effect evaluation of any new drug.

#5




Well, C & E are more remote possibilities...we dont have all the details about the actual design...but assuming same follow up time and 0 drop out, if you have a p=0.1, maybe if you increase your power (decreasing your type II error) w/ same preset alpha you would actually get an stat. significant difference between groups

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iron (11112011) 
#6




Quote:

#7




Quote:
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you are born with wings, don't walk through life To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. Last edited by busterbee; 11112011 at 10:33 AM. 
#8




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D = drug n=50 no D = no drug n=50 Follow up = 1 year No drop out = everybody was measured = no attrition Preset alpha (probability type I error = Probability of reject H0 when H0 is true) = 0.05 Analysis group D vs no D diff. in secondary outcome with p=0.10 "General formula" for sample size calculation: n = [alpha * beta * variability]/[minimum significant difference]  actually we use the Z value of alpha & beta...like alpha = 0.05 ==> Z = 1.96  and variability we use SD (most of the time)  low probability ==> greater Z score So, keeping alpha, variability and min.sig. diff. equal, if we decreased beta, Z score goes up ==> sample size goes up ==> the chances of detecting an smaller min.sig.diff goes up. 
#9




Quote:
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iron (11112011) 
#10




Well, alpha & beta probabilities are numbers selected by the researcher
alpha almost always is preset = 0.05 range of beta = 0.10 to 0.20 
#11




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THANKS
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#13




same here.... I was thinking my biostat has improved but seeing this question I think I will again go into depression, will come to this question once again after revising biostat..

#14




YOU DONT have to know this formula....and btw, this is only the general formula (just to explain a concept)...they never going to ask u this!!!
Here is the actual formula http://en.wikipedia.org/wiki/Sample_size_determination 
#15




Quote:
seriously after all the assessments i did i gave time to biostat n still dont cant get it..so its all left to fate
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#16




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#17




[QUOTE=busterbee;78379]you know me..i get it n forget it..its easy but only once you get it
seriously after all the assessments i did i gave time to biostat n still dont cant get it..so its all left to fate[ I know, just don't worry abt it anymore, do well in other sectors. 
#18




[QUOTE=iron;78382]thankyou for your support guys...i will ace...INSHALLAH
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