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#1




Biostatistics #5  Interpreting a normal curve
The IQs of a class of students attending a university are distributed according to the normal curve, with a mean of 115 and a standard deviation of 10. Therefore:
a. 50% have IQs between 105 and 115. b. 95% have IQs between 105 than 115. c. 2.5% have IQs above 135. d. 5% will have IQs above 135. e. 5% have IQs below 95. 
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eveli55 (02022013) 


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#7




shouldn't it be 125??

#8




If the mean IQ is 115 & the SD is 10
Mean + 2 SD = 135 Mean  2SD = 95 then, above 135 = 2.5% and below 95 = 2.5% 
#9




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range at 95% will be 2SD 2*10 = 115 +/20, will be between 95135 (NOT between 105115) range at 99% ( after 2SD:95%, remaining total percent is 5% = 3SD or 2.5% EACH side): means 2.5% will have IQ below 95 and 2.5% will have IQ above 135. because between 95135 is 2SD (95%). so if you go below 95 or if you go above 135, you will go into 3SD (99%) range. therefore, only option C matches the results. 
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Aspirations are good, untill it's gastric. 
#11




Can anyone please explain this question
A new serologic test for detecting prostate cancer is negative in 95% of the patients who do not have the disease. If the test is used in eight consecutive blood samples taken from patients without the disease, what is the probability of getting at least one positive test result ?
A. 10.05 * 8 B. 0.05 * 8 C. 0.05^8 D. 0.95^8 E. 10.95^8 (^ = raised to the power of ) 
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My answer is E. Here's why. 1. Negative in 95% = Specificity 2. Used 8 samples 3. What's probability of getting all negative? All negative = 0.95^8 4. What's probability of getting at least 1 positive? That would be 1  Probability all negative = 1  0.95^8. 5. My answer is E. 
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Thrax_usmle (03112013) 
#13




I would go with B.
As the probability of getting negative test result is 95%, the probability of getting a positive test is 5%. As we should get atleast 1 positive result out of 8 samples which is a mutually exclusive event, 0.05 * 8 would be the correct answer. If the question says all the 8 results should come positive then the answer would be 0.05 ^ 8. I am not so sure. If somebody know the answer correctly please explain........ 
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i thnk the answer wud b option B. 
#15




so when they say "a standard deviation" they mean its only 1 standard deviation correct?

#16




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Theres a UWorld question like this.... I can't find it but does anything know the question # by chance?? 
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In USMLE, “Any fool can know. The point is to understand.” 
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yes, you're correct 
#19




why is it E? I dont get it ...

#20




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so the formula is; 1specificity = false positive result. further answer is derived above! 
#21




ah i see, yeah that makes sense now. 8 blood samples from people who dont have the disease, which means true negative (i.e specificity). 95 percent of the time the test is negative... the remaining 5 percent will test positive, but its a false positive, obviously.

#22




i completely agree … that's the ans

#23




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1specificity=fp probability since testing pts eight times are independent events,multiply the probabilities so it comes out to b (10.95) or ).05^8 …(b is not the and bcos * means multiplying by 8 ) 
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