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NBME 24 Answers

nbme24/Block 4/Question#30
An investigator is studying an outbreak of ...
Only cookies are independently associated with E. coli cases🔍

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 +25  upvote downvote
submitted by niboonsh(220),

this question makes me want to eat an e coli cookie and hope i bleed out

 +12  upvote downvote
submitted by doodimoodi(39),

Did no one notice that the Odds ratio on the top left is wrong? Am I missing something? If you calculate it, it's 6 just like the top right one....

mjmejora  thats actually really funny +  
yex  Because I said so, applies here... :-/ +  
doodimoodi  Cant believe we pay $60 for this crap +23  
aisel1787  best comment doodimoodi) +1  
b1ackcoffee  that fucking threw me off on exam. I was like is there an effect modification by "Not drinking milk". the fuck! +  

 +5  upvote downvote
submitted by pg32(64),

The fact that the odds ratio in the top left is incorrect makes this question very difficult. It makes it appear as if the cookies are causative but the milk had some protective factor. So obnoxious.

drpatinoire  God I thought totally the same way as you did. I stared at this question for at least 5min and asked myself what's wrong with my statistics. +1  

 +3  upvote downvote
submitted by youssefa(69),

Initially milk drinking was associated with E.coli outbreak with OR=3.9 and P<0.001 (Significant)... After stratification into ate cookies and did not eat cookies OR became 1 instead of 3.9 meaning the association disappeared. Therefore, eating cookies was a confounder and there is no real association between drinking milk and E.coli....instead, milk's (the confounder) contribution was responsible for the OR of 3.9 in the first place. This was furthered demonstrated with OR of 6 in the cookies alone group.

 +2  upvote downvote
submitted by sne(26),

OR >1 indicates increased occurrence of event. The only OR greater than 1 was in the table that indicated that the subject ate cookies but didn't drink milk. Thus, that is the only one with a significant occurrence

For a more systematic approach. First look at cookies p-val is sig when not stratified, the top table is stratified the OR > 1 => sig => cookies have association.

Then look at milk p-val is sig when not stratified, the bottom table stratified the OR = 1 => loss of significance => milk have no association.

+/- peqmd(10),

Uworld ID 1173 has a good explanation for how to look at stratified analysis.

+/- peqmd(10),

 +1  upvote downvote
submitted by famylife(61),

"An odds ratio of 1 indicates that the condition or event under study is equally likely to occur in both groups. An odds ratio greater than 1 indicates that the condition or event is more likely to occur in the first group." (https://en.wikipedia.org/wiki/Odds_ratio)

 +1  upvote downvote
submitted by sattanki(50),

This one there were four odds ratios, one provided under each table. The only one that had an odds ratio greater than 1.0 was the table in the top right (Odds Ratio = 6, I believe), which when you looked at the labels, led to the right answer.

 +0  upvote downvote
submitted by qiss(0),

For people who generally had trouble reading the two charts:

First chart: We separated the entire population into two smaller populations to test for the cookies affect. In Population A (drank milk) there was an odds ratio of 6 (typo in the actual chart). In Population B (did not drink milk) there was an odds ratio of 6. Since the odds ratios are not 1, we can conclude that the cookies have an effect regardless of the population (ie drank milk people versus didn't drink milk people).

Second chart: New set of populations to test for the effect of milk. In Population C (ate cookies) there was an odds ratio of 1. In Population D (did not eat cookies) there was also an odds ratio of 1. This means that milk did not have an effect ever and didn't contribute to the disease.

"Only cookies are independently associated with E. coli cases" means that only the cookies cause the disease without the effects of something else.

 -5  upvote downvote
submitted by lilmonkey(16),

The keyword is "INDEPENDENTLY"(associated). Which in human language means "NOT ASSOCIATED".

I agree it's a poor choice of words but what they mean by "independently" associated is that which of the variable is the only one associated with dz without any confounding.

+/- peqmd(10),

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