Biostatistics #5 - Interpreting a normal curve - USMLE Forums
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  #1  
Old 06-02-2011
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Stats 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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  #2  
Old 06-02-2011
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Default C

C. 2.5 % IQ above 135
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  #3  
Old 06-02-2011
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I'll go with c
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  #4  
Old 06-02-2011
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Quote:
Originally Posted by bebix View Post
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.
My answer is C)
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  #5  
Old 06-02-2011
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Quote:
Originally Posted by bebix View Post
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.
yup its C....
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  #6  
Old 06-03-2011
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CORRECT,
the answer is "2.5% have IQs above 135."
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  #7  
Old 06-03-2011
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shouldn't it be 125??
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  #8  
Old 06-03-2011
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Correct Answer

Quote:
Originally Posted by earthpole View Post
shouldn't it be 125??
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%
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  #9  
Old 02-01-2013
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Quote:
Originally Posted by bebix View Post
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.
mean 115 +/-10(which is 68%= 1SD) means the range is between 105-125
range at 95% will be 2SD 2*10 = 115 +/-20, will be between 95-135 (NOT between 105-115)
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 95-135 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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  #10  
Old 02-01-2013
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Quote:
Originally Posted by jatminder View Post
mean 115 +/-10(which is 68%= 1SD) means the range is between 105-125
range at 95% will be 2SD 2*10 = 115 +/-20, will be between 95-135 (NOT between 105-115)
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 95-135 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.
Congrats you r FIRST
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  #11  
Old 03-11-2013
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Question 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. 1-0.05 * 8
B. 0.05 * 8
C. 0.05^8
D. 0.95^8
E. 1-0.95^8

(^ = raised to the power of )
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  #12  
Old 03-11-2013
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Quote:
Originally Posted by Thrax_usmle View Post
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. 1-0.05 * 8
B. 0.05 * 8
C. 0.05^8
D. 0.95^8
E. 1-0.95^8

(^ = raised to the power of )



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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  #13  
Old 11-24-2013
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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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  #14  
Old 11-27-2013
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Quote:
Originally Posted by Thrax_usmle View Post
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. 1-0.05 * 8
B. 0.05 * 8
C. 0.05^8
D. 0.95^8
E. 1-0.95^8

(^ = raised to the power of )
correct answer plz...!

i thnk the answer wud b option B.
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  #15  
Old 11-27-2013
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so when they say "a standard deviation" they mean its only 1 standard deviation correct?
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  #16  
Old 11-27-2013
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Quote:
Originally Posted by Thrax_usmle View Post
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. 1-0.05 * 8
B. 0.05 * 8
C. 0.05^8
D. 0.95^8
E. 1-0.95^8

(^ = raised to the power of )


Theres a UWorld question like this.... I can't find it but does anything know the question # by chance??
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  #17  
Old 11-28-2013
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Quote:
Originally Posted by Thrax_usmle View Post
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. 1-0.05 * 8
B. 0.05 * 8
C. 0.05^8
D. 0.95^8
E. 1-0.95^8

(^ = raised to the power of )
this is uworld question answer is E
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  #18  
Old 11-28-2013
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Quote:
Originally Posted by CisternaChyli View Post
so when they say "a standard deviation" they mean its only 1 standard deviation correct?

yes, you're correct
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  #19  
Old 11-28-2013
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why is it E? I dont get it ...
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  #20  
Old 11-28-2013
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Quote:
Originally Posted by Thrax_usmle View Post
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. 1-0.05 * 8
B. 0.05 * 8
C. 0.05^8
D. 0.95^8
E. 1-0.95^8

(^ = raised to the power of )
we just misinterpreted the question, if read the question properly, we see, it is asking for a false positive result.
so the formula is; 1-specificity = false positive result.
further answer is derived above!
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  #21  
Old 11-29-2013
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Quote:
Originally Posted by organo14 View Post
we just misinterpreted the question, if read the question properly, we see, it is asking for a false positive result.
so the formula is; 1-specificity = false positive result.
further answer is derived above!
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.
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  #22  
Old 12-11-2013
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Quote:
Originally Posted by bebix View Post
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%
i completely agree … that's the ans
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  #23  
Old 12-11-2013
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Quote:
Originally Posted by organo14 View Post
we just misinterpreted the question, if read the question properly, we see, it is asking for a false positive result.
so the formula is; 1-specificity = false positive result.
further answer is derived above!
95 % specificity
1-specificity=fp probability
since testing pts eight times are independent events,multiply the probabilities so it comes out to b (1-0.95) or ).05^8 …(b is not the and bcos * means multiplying by 8 )
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