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NBME 21 Answers

nbme21/Block 4/Question#40 (45.0 difficulty score)
A study is conducted to assess the normal ...
500 Men from a list of patients scheduled to be examined by a urologist🔍

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 +16  upvote downvote
submitted by seagull(1080),
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forerofore  to add up, the urologist himself doesn't add or remove accuracy (since this is a blood test), what decreases the accuracy is the fact that in order to be sent to a urologist you probably are sick in the first place (selection bias), so your urea nitrogen is likely to be altered. +18  
sharpscontainer  I thought of precision as more of a function of variance. Variance will decrease with a greater sample size. Had a hard time because I was thinking about those 4 darn targets (wouldn't 500 darts look more spread out than 10? but no, the variance will be better) that have been in my textbooks since 7th grade and for the first time I was asked a question about this concept only to discover that I didn't have it down as well as I assumed. +1  
peridot  @sharpscontainer I feel you, I thought the exact same thing. Looked into it a bit and I think it has something to do with the way standard error or standard deviation or something like that is calculated, but I'm still confused and too tired to dig further. Also, wanted to mention that this NBME has a similar question but instead it's about the 95% confidence interval - maybe that'll help you understand the precision thing better since the 95% confidence interval narrows with a larger sample size? So it's kinda tied to precision? +  

 +10  upvote downvote
submitted by usmleuser007(321),
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 +1  upvote downvote
submitted by drdoom(615),
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rHese’ a enci emai:g

Discussing precision only makes sense if they were to sample "X # patients" multiple times and see how close the different measurements' results were to each other. The actual size of the sample should't affect precision, but rather it should just affect accuracy (which is reduced by the biased population at the urologist). Smh