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#1
08-04-2012
 USMLE Forums Guru Steps History: Not yet Posts: 414 Threads: 30 Thanked 86 Times in 69 Posts Reputation: 96
standard deviation question

A particular association determines membership based on members' IQ scores. Only those people who have documented IQ scores at least two standard deviations above themean on the Wechsler Adult Intelligence Scale, Revised (WAIS-R), are eligible for admis*sion. Out of a group of 400 people randomly selected from the population at large, howmany would be eligible for membership to this society?
(A) 2
(B) 4
(C) 6
(D) 8
(E) 10
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#2
08-04-2012
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#3
08-04-2012
 USMLE Forums Scout Steps History: 1+CK+CS+3 Posts: 26 Threads: 7 Thanked 86 Times in 13 Posts Reputation: 96
2.5%, that's the key.

I think the answer is 10. 95% of the population will be in the 2 SD straddling the mean...so 2.5% will remain on each side of that 95% distribution.

Hence the answer = 2.5/100 * 400 = 10.

I am surprised that one of the options is not 20, that would have been picked if you forgot that it was 2.5% not 5%.

I hope this helps.
 The above post was thanked by: Doc4Step1 (08-04-2012)

#4
08-04-2012
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I agree - (E) 10
#5
08-04-2012
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Of course 10 will be the straight choice but if ans had option 20 than ?
The question said two SD above that leaves 5% ?
__________________
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#6
08-04-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 557 Times in 318 Posts Reputation: 567
.

Quote:
 Originally Posted by Kabutar111 Of course 10 will be the straight choice but if ans had option 20 than ? The question said two SD above that leaves 5% ?
I'm not sure if I understand...
But if the question says 2 SD above it leaves us with 2,5%.
2 SD below and 2 SD above would give 5%.
#7
08-04-2012
 USMLE Forums Guru Steps History: Not yet Posts: 414 Threads: 30 Thanked 86 Times in 69 Posts Reputation: 96

Well that is my question 2 SD above is 3rd SD that aprox 5% .
If 10 is the answer expected than language wrong or this is the format used for SD questions ??
__________________
There is no shortcut to SUCCESS ,and there is no excuse for FALIURE - by Kabutar111
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#8
08-04-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 557 Times in 318 Posts Reputation: 567
.

Quote:
 Originally Posted by Kabutar111 Well that is my question 2 SD above is 3rd SD that aprox 5% . If 10 is the answer expected than language wrong or this is the format used for SD questions ??
There's nothing wrong with this question!
If this is 2 SD above it's 2,5% NOT 5%.

Take a look at the chart here: http://www.google.pl/imgres?imgurl=h...9QEwAg&dur=155

2 SD above is 2,5% (in the pic they say it's 2,1% but ignore this difference) bc you're looking at what is to the right of the 2 SD line.

if you take a look at what is to the left of -2 SD you will see another 2,5%.
SO when you sum it up it gives you 5%. But only when you sum it up! Not when you read one side of the chart only.

 Tags Biostatistics-Epidemiology, Step-1-Questions

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