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#1
12-27-2012
 USMLE Forums Addict Steps History: CK+CS Posts: 138 Threads: 28 Thanked 78 Times in 36 Posts Reputation: 88
Calculating the Standard of Deviation (SD)

A survey of a popular seaside community (population =1,225) found the local inhabitants to have unusually elevated blood pressures. In this survey, just over 95% of the population had systolics between 110 and 190. Assuming a normal distribution for these assessed blood pressures, the standard deviation for systolic blood pressure in this seaside community is most likely:

(A) 10
(B) 20
(C) 30
(D) 40
(E) 50

#2
12-27-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

(B) 20..................
#3
12-27-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 465 Times in 298 Posts Reputation: 475

Agree
mean-150
SD-20
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#4
12-28-2012
 USMLE Forums Scout Steps History: 1+CK+CS+3 Posts: 56 Threads: 6 Thanked 16 Times in 11 Posts Reputation: 26
help

Quote:
 Originally Posted by MedicalExaminer Agree mean-150 SD-20
plz elaborate how did u calculate that... thanks
#5
12-28-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 465 Times in 298 Posts Reputation: 475

Quote:
 Originally Posted by Wajahat plz elaborate how did u calculate that... thanks
Ok
First of all u need to find a mean
Here u have givven smallest and largest values. To find mean u add them and devide by 2 = 110+190 / 2 =150

95% of population is in 2SD on normal (Gaussian) distribution

Now as u know the mean=150
U should find a number(SD) which when added twice(+2SD) to the mean gives u 190 and when subtracted twice(-2SD) from mean(150) gives u 110
__________________
Everything is possible for him who believes (MARK 9:23)
245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad
 The above post was thanked by: Doc4Step1 (12-28-2012)
#6
12-28-2012
 USMLE Forums Guru Steps History: Not yet Posts: 375 Threads: 17 Thanked 131 Times in 81 Posts Reputation: 147

Quote:
 Originally Posted by MedicalExaminer Ok First of all u need to find a mean Here u have givven smallest and largest values. To find mean u add them and devide by 2 = 110+190 / 2 =150 95% of population is in 2SD on normal (Gaussian) distribution Now as u know the mean=150 U should find a number(SD) which when added twice(+2SD) to the mean gives u 190 and when subtracted twice(-2SD) from mean(150) gives u 110
I couldn't stop myself from posting a reply and thank you.

This is the first time in the last 1 year that I understood this stupid calculation....i always thought I would skip this question if I ever get one on the real exam....But now looks like i will be looking for these questions on my exam...I really thank you for that...Wish you the best.

Just to make sure I got this right.So e.g. If just over 68% of the population had systolic between 110 and 190, The answer would be , 40 cos of 1 SD????

and if the 99.7% of the population had systolic between 110 and 190, the answer would be 13.333 cos 3SD?????

I hope I am right...But thanks anyway.
 The above post was thanked by: MedicalExaminer (12-28-2012), podebrad (12-28-2012)
#7
12-28-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 465 Times in 298 Posts Reputation: 475

Quote:
 Originally Posted by Doc4Step1 I couldn't stop myself from posting a reply and thank you. This is the first time in the last 1 year that I understood this stupid calculation....i always thought I would skip this question if I ever get one on the real exam....But now looks like i will be looking for these questions on my exam...I really thank you for that...Wish you the best. Just to make sure I got this right.So e.g. If just over 68% of the population had systolic between 110 and 190, The answer would be , 40 cos of 1 SD???? and if the 99.7% of the population had systolic between 110 and 190, the answer would be 13.333 cos 3SD????? I hope I am right...But thanks anyway.
U are welcome fellow

Yes
thats the way processing it

__________________
Everything is possible for him who believes (MARK 9:23)
245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad
 The above post was thanked by: Doc4Step1 (12-28-2012)
#8
12-28-2012
 USMLE Forums Addict Steps History: CK+CS Posts: 138 Threads: 28 Thanked 78 Times in 36 Posts Reputation: 88

I am not posting the answer and explanation. All of you have done a good job at solving it.

And it was a question given in Kaplan statistics. That chapter has some nice and interesting questions!!

 Tags Biostatistics-Epidemiology, Step-1-Questions

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