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NBME 20 Answers

nbme20/Block 1/Question#26

Serum cholesterol concentrations are measured as ...

16%

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So you know that 65% of the data will fall within 1SD of the mean. So if you subtract 100-65 you will get 35. Which means that about 16% will fall above and 16% will fall below 1 SD. They are asking for how many will fall above 1 SD. I'm sure there is a better way of doing this, but thats how I got it lol.

sympathetikey  Same! +  
sympathetikey  Except according to FA, it's 68% within 1 SD, so 34%, which split in half is 17%. +2  
amirmullick3  Sympathetikey check your math :D 100-68 is 32 not 34, and half of 32 is 16 :) +2  
lilyo  Can anyone explain why we subtract 68 from 100? This makes me think that we are saying its 35% of the data that falls within 1SD as opposed to 65. HELLLLLLP +  
sallz  @Lilyo If you consider 1 SD, that includes 68% of the population (in this case, you're saying that 68% of the people are between 296 and 196 (1SD above and 1 below). This leaves how many people? 32% outside of that range (100-68=32); half of those would be above 296 and the other half below 296, so 16% +  




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submitted by smpate(2),

To get this one right, you would have to know that one standard deviation away from the mean on a bell curve is 34% on either side. After that it would be another 13.5% (but memorizing that is low yield). Looking at the specified age group, we want to know how much is greater than the mean plus 1 standard deviation. Therefore, 50% (the mean) plus 34% (1 standard deviation) is 84%. The rest is 100-84 = 16.

I found this image online for clarity

http://www.mathandstatistics.com/wp-content/uploads/2014/06/Example1EmpiricalGraphNormal.jpg