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#1
10-08-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 623 Threads: 111 Thanked 424 Times in 264 Posts Reputation: 434
Calculating from Standard Deviation and Mean

A study of 400 patients hospitalized with diabetes mellitus related complications shows that the serum cholesterol level is a normally distributed variable with a mean value of 220mg/dl and standard deviation of 10 mg/dl. Based on study result, how many patients do you expect to have serum cholesterol level > or equal to 240mg/dl in this study??

a) 2
b) 10
c) 20
d) 64
e) 120

Please reply with explanation

#2
10-08-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 3,317 Threads: 170 Thanked 4,880 Times in 1,808 Posts Reputation: 4933

Use this
Normal Gaussian Distribution

It will help you in coming up with the answer

-
 The above post was thanked by: aknz (10-08-2011)
#3
10-08-2011
 USMLE Forums Scout Steps History: --- Posts: 76 Threads: 9 Thanked 87 Times in 25 Posts Reputation: 97
....

a)2

i've seen this question somewhere
#4
10-08-2011
 USMLE Forums Addict Steps History: Step 1 Only Posts: 145 Threads: 8 Thanked 75 Times in 57 Posts Reputation: 85

Ans is b) 10

Explanation: Mean is 220, so you keep the center value 220. SD is 10, so >or=240 will lie above 2 SD.

As you know -1 to +1 = 68, and -2 to +2 = 95 and -3 to +3 = 99.7 per Gaussian Distribution. And as a whole will be100% ofcourse.

So calculation --> 50%(Half of 100%) - 34(half of 68) - 13.5(half of 27) =2.5%

This means 2.5% of people have BS >or= 240.

We have total pt = 400. SO, no. of pt having BS >or= 240 === 2.5% of 400. i.e. 10

Hope I was clear.

Last edited by whocares13; 10-08-2011 at 04:11 PM.
 The above post was thanked by: aknz (10-08-2011), rahul (10-08-2011)

 Tags Biostatistics-Epidemiology, Step-1-Questions

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