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#1
06-06-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269
Biostatistics #13 - Probability and cholesterol

Serum cholesterol levels are obtained for four healthy men. The probability that all will fall below the 10th percentile of the distribution of cholesterol for healthy males is:

a. 0.4
b. 1 - (0.1)^4
c. (0.1)^4
d. (0.9)^4
e. cannot be determined from these data

x^y = x to the power of y

* I did a pretty similar question in "UW"...
 The above post was thanked by: dr_lizard (09-01-2011), rulz (06-06-2011)

#2
06-06-2011
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 The above post was thanked by: bebix (06-06-2011)
#3
06-06-2011
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#4
06-06-2011
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Quote:
 Originally Posted by bebix Serum cholesterol levels are obtained for four healthy men. The probability that all will fall below the 10th percentile of the distribution of cholesterol for healthy males is: a. 0.4 b. 1 - (0.1)^4 c. (0.1)^4 d. (0.9)^4 e. cannot be determined from these data x^y = x to the power of y * I did a pretty similar question in "UW"...
The aproximation of this is to take the real total minus the percentile of distribution.

 The above post was thanked by: bebix (06-06-2011)
#5
06-06-2011
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 406 Threads: 30 Thanked 140 Times in 108 Posts Reputation: 150

i think its C??
 The above post was thanked by: bebix (06-06-2011)
#6
06-07-2011
 USMLE Forums Newbie Steps History: Step 1 Only Posts: 8 Threads: 0 Thanked 4 Times in 4 Posts Reputation: 14

I think it's C
Each of them has 10% probability to fall below 10th percentile, so the probability that all of them are below that limit is (0.10)*(0.10)*(0.10)*(0.10), thus (0.10)^4.
#7
06-07-2011
 USMLE Forums Guru Steps History: Not yet Posts: 324 Threads: 70 Thanked 236 Times in 143 Posts Reputation: 246

Quote:
 Originally Posted by bebix Serum cholesterol levels are obtained for four healthy men. The probability that all will fall below the 10th percentile of the distribution of cholesterol for healthy males is: a. 0.4 b. 1 - (0.1)^4 c. (0.1)^4 d. (0.9)^4 e. cannot be determined from these data x^y = x to the power of y * I did a pretty similar question in "UW"...
i will go with C.
probability of four persons falling below 10th percentile =(0.1)^4
#8
06-07-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

Here we could have 2 answers...

Since they did not mention that the distribution of cholesterol for healthy males follows a normal distribution, one could say that the answer is E). (answer from the book)

Now, if we assume that the underlying distribution of cholesterol is normal (i did), the answer will be C)

Alternative B) is the probability that ALL will fall above 10th p.

Thanks!
 The above post was thanked by: pass7 (06-07-2011), rulz (06-07-2011)

 Tags Biostatistics-Epidemiology, Step-1-Questions

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