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#1
06-02-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269
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.
 The above post was thanked by: eveli55 (02-02-2013)

#2
06-02-2011
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C

C. 2.5 % IQ above 135
 The above post was thanked by: bebix (06-02-2011), benzo (06-03-2011)
#3
06-02-2011
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I'll go with c
 The above post was thanked by: bebix (06-02-2011)
#4
06-02-2011
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Quote:
 Originally Posted by bebix 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)
#5
06-02-2011
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Quote:
 Originally Posted by bebix 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....
#6
06-03-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

CORRECT,
the answer is "2.5% have IQs above 135."
 The above post was thanked by: benzo (06-03-2011), mle guy (06-03-2011), pass7 (06-03-2011)
#7
06-03-2011
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shouldn't it be 125??
#8
06-03-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

Quote:
 Originally Posted by earthpole 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%
#9
02-01-2013
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Quote:
 Originally Posted by bebix 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.
#10
02-01-2013
 USMLE Forums Addict Steps History: 1+CK+CS+3 Posts: 103 Threads: 6 Thanked 102 Times in 40 Posts Reputation: 112

Quote:
 Originally Posted by jatminder 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
03-11-2013
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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 )
#12
03-11-2013
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 283 Threads: 35 Thanked 204 Times in 115 Posts Reputation: 214

Quote:
 Originally Posted by Thrax_usmle 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.
 The above post was thanked by: Thrax_usmle (03-11-2013)
#13
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........
#14
11-27-2013
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Quote:
 Originally Posted by Thrax_usmle 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 )

i thnk the answer wud b option B.
#15
11-27-2013
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 367 Threads: 0 Thanked 70 Times in 60 Posts Reputation: 80

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

#16
11-27-2013
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Quote:
 Originally Posted by Thrax_usmle 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??
#17
11-28-2013
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,109 Threads: 31 Thanked 520 Times in 394 Posts Reputation: 539

Quote:
 Originally Posted by Thrax_usmle 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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In USMLE, “Any fool can know. The point is to understand.”
#18
11-28-2013
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 198 Threads: 26 Thanked 174 Times in 79 Posts Reputation: 184

Quote:
 Originally Posted by CisternaChyli so when they say "a standard deviation" they mean its only 1 standard deviation correct?

yes, you're correct
#19
11-28-2013
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 367 Threads: 0 Thanked 70 Times in 60 Posts Reputation: 80

why is it E? I dont get it ...
#20
11-28-2013
 USMLE Forums Scout Steps History: Not yet Posts: 11 Threads: 0 Thanked 6 Times in 4 Posts Reputation: 16

Quote:
 Originally Posted by Thrax_usmle 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!
#21
11-29-2013
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 367 Threads: 0 Thanked 70 Times in 60 Posts Reputation: 80

Quote:
 Originally Posted by organo14 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.
#22
12-11-2013
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 32 Threads: 14 Thanked 1 Time in 1 Post Reputation: 11

Quote:
 Originally Posted by bebix 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
#23
12-11-2013
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 32 Threads: 14 Thanked 1 Time in 1 Post Reputation: 11

Quote:
 Originally Posted by organo14 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 )

 Tags Biostatistics-Epidemiology, Step-1-Questions

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