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NBME 21 Answers

nbme21/Block 4/Question#40 (49.9 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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 +18 
submitted by seagull(1436),
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gnnaEiixm netpiat frmo a girstoolu ismiepl sonrkBe asiB hicwh louwd kesw teh ooultpapni mean of esurm raue irneotgn ayaw ofmr hte rtue atcearcu an.em hT,en aeizlre npiicoser si endeendtp on sitalcstati ""Peowr ihhwc is ncrdesiae badse no the sezi fo eth topuanopli fo het sdu.ty (earcidsne riopnisce = areecnsdi tiaiastclst )porew. heerTof,re na enriacse in auniootppl of a eabsdi pugro htiw alde to anycrcaicu tiwh ighh ciirn.sepo

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. +21  
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? +  



 +11 
submitted by usmleuser007(377),
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.1 lxpeEam of raicuneatc but glhhiy peecris 
    a. 500 tnetspia iseeng a pclaurarti cdorot ofr a uarpcatlri s
s2ei.lln apEmexl fo ercutcaa ubt iscr
mepie    a. 01 etpsatin eogurnd a rgeenscni at a mlla 
3. ohBt cceuArta and creespi 
    a. 005 atpistne gi(hh erip)sicon ogundre a nensgeric g(hhi ayccuarc ~ no sabi or sstyecmi )
orrre



 +1 
submitted by drdoom(819),
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cyarcAcu mnase het aatd tniosp ear ,derpsseid btu hwen yuo taek eth anme of stheo ,otspni tath eman asepm“l( )m”nae si anreyb eth oatppiuoln mena ur(t“e a.)me”n Data ntpios rae m“ero ”ecerisp if the riinosdspe sasroc data opsnit si eralslm ntha soem eorth ets fo dtaa tipnso ctie(no ohw tshi is a macinsproo dan not na u”ae“tlbos at;teem)tsn osiicenpr sasy hotinng tubao ohw seolc hte reagave fo hte atda ptsoni ear to the e“utr a”mne.

eKpe ni imdn ahtt cacrcyua and sciornpei are aveiletr rrot;cspeids uoy anct’ ays dos“--oans si ipcrsee”; ,on ouy nac ynol ysa cuas“-nhsh-ucd is mero rpeesic hnta doason”--s ro o-sdan-o“s si reom racatuce hnta s.--”uhhcuasdnc ,oS in ihts asec, we nca ienrf ahtt EBNM seornscid m“ne ta eht ”sugoiotrl to vhea NUBs htat rea colser ot cahe toerh oer(m cdstreel;u meor ;rsceeip lsse dp)erdisse naht hte NUBs of “men at m.al”l

ees’rH a ceni emi:ga
c/ems/ccslnaieeresa/jogyrp..tmsitdcotumphi/bs--p:vagclu




 +0 
submitted by cuthbertallg0od(11),

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