








USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam 

Thread Tools  Search this Thread  Display Modes 
#1




The likelihood that the difference is due to chance
two experimental drugs are being researched for the treatment of CHF
patients getting drug X have a cardiac index of 2.5L/m with a 95% confidence interval between 1.5 to 3.5. patients getting drug y have cardiac index with 95% confidence interval between 0.7 to 3.7. a teat of significance of difference shows a pvalue of 0.1. which of the following is the likelihood that the difference in mean cardiac index of patients getting drugs x and y is due to chance: a.0% b.2.5% c.5% d.7.5% e10% f.66.7% g.95% i would really appreciate a concise explanation as i have no idea what they are talking about,i have read statistics in FA thanks 
#2




which of the following is the likelihood that the difference in mean cardiac index of patients getting drugs x and y is due to chance:
This is the same as saying "we fail to reject H0" H0 = difference =0 HA = difference =! 0 So, you preset an alpha value of 5% (0.05) Then you did the test statistics under the null hypothesis and you got a p value = 0.10 (10%) = posttest Conclusions = you fail to reject the null hypothesis When the pvalue is < 0.05, reject the null hypothesis. With such a low probability for the pvalue, there is little likelihood that the observed difference between the sample mean and hypothesized mean is due to chance  it must be do to some program, process change, intervention or other effect. When the pvalue is > 0.05, fail to reject the null hypothesis. There is a high probability for the pvalue that the observed difference between the sample mean and the hypothesized mean is so small that it must be do to chance involved in sampling error. In this case, the answer is 10% 
The above post was thanked by:  
dinosaur108 (08302011) 
#3




Here difference in mean is due to chance is null hypothesis.
Now, p = probability of making alpha error or Rejecting the null hypothesis when it was true. = 0.1 = 10% When p>0.05 we do not reject null hypothesis. So yeah difference was definitely due to chance and 10% chance were there we may have rejected it. And also 10% is likelihood of difference is due to chance!!!!!! 


#4




Quote:
the probability of a Type I error here is equal to alpha = 0.05 alpha is NOT the same as pvalue (in this case 0.10) 
#5




thanks
thanks for the explanations

#6




This is NBME form 3 question..
Posting NBME qs is not allowed. 
#7




Quote:
alfa error is p value  .1 in this case 
#8




Quote:
alpha is the probability of a type I error. In this case is a PREtest probability equal to 0.05 (something that an investigator can manipulate BEFORE do the actual experiment) (probability alpha = Probability to reject the Null given the Null is true) pvalue is a POSTtest probability...you cant change this...is is the probability that you get with the data that you have in that particular time! (in this case 0.1) The interpretations are completely different. So first you preset an alpha = 0.05....then you do the test statistic and you get a pvalue...if this pvalue is less than you preset alpha, you'll reject the null hypothesis (there is little likelihood that the observed difference between the sample mean and hypothesized mean is due to chance) 
#9




Quote:

#10




Quote:
here we have a 95% CI...so, we are talking about population parameters ("mu" and not "mu hat")....we are 95% confident that the population parameter mu lies between the lower and the upper bound... 
#11




Quote:
when there is failure to reject null hypothesis (P>5%) as here which is 10% chance of alfa error is 0 bcz we dont put that drug in practical life.............but what is the chance of beta error( difference in mean cardiac index of patients may be due to drug and not due to chance)? what is the power ? 
#12




Quote:
Type 1 error = reject the null given the null is true Alpha = probability of a type 1 error In this case: type 1 error (PRESET) = 0.05 pvalue (POST)= 0.10 The term significance level (alpha) is used to refer to a prechosen probability and the term "P value" is used to indicate a probability that you calculate after a given study. Beta again is something that you must decide before you run your experiment (since youŽll need that value and the alpha value to calculate the sample size). Typically we use beta = 0.1 (power = .9) or 0.2 (power = 0.8). Here you can find more info: http://www.statsdirect.com/help/basics/pval.htm if you want, you can send me a pm. 
The above post was thanked by:  
dinosaur108 (08312011), drnrpatel (08302011) 
Tags 
BiostatisticsEpidemiology, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Likelihood Ratio  kalpana  USMLE Step 1 Forum  2  02172016 10:24 AM 
The likelihood that he has the disease?  thunderbolt  USMLE Step 2 CK Forum  15  05172011 09:28 AM 
The likelihood that the patient is having cancer  achistikbenny  USMLE Step 1 Forum  4  06292010 12:13 AM 
Step 3 and Residency Matching Likelihood  Steve13  USMLE Step 3 Forum  2  04192010 11:08 AM 
do u think I have chance?  yellowsand  IMG/FMG Forum  4  10112009 11:15 AM 
