HAHAHAHAHA
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This is a great example of problematic epidemiology reporting, so strap in and let's have some fun #epitwitterhttps://twitter.com/MailOnline/status/1108284477128744960 …
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Now, we can't calculate the adjusted rates of cancer based on the info in the study. We'd have to run their regression model for that But the crude rates will give us some information here
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The crude rate in the cold tea group = 92/19,450 = 0.47% In the hot tea group = 87/10,799 = 0.81% The unadjusted relative risk ratio is ~1.70 But what about the ABSOLUTE risk difference?pic.twitter.com/S8Oa9volte
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Remember, relative risk ratio is p(1)/p(2) = 0.81/0.47 = 1.7 = 70% increased risk! Absolute risk difference is p(1)-p(2) = 0.81-0.47 = 0.34 = 0.34% increased risk THIS MAKES THE RESULTS MUCH LESS MEANINGFUL
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(Yes, the answer above was that they used relative risk which is not a great measure when the risk of an event is small)
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So, we've established that the increased risk of cancer was more like 0.34% from drinking very hot tea, every day, for 10 years But that's not the only issue here!pic.twitter.com/POedaTnAWP
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Firstly, how did they collect tea heat preferences? It's not a variable you normally think about
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The researchers got their subject to sit in a room, and gave them a cup of tea prepared at 75 degrees They put another cup of tea in front of them with a thermometer in it, and at cooling intervals asked the participants to take a sip and see if it was their preferred temppic.twitter.com/dTcdO2NoTQ
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Now, I don't necessarily have a better way to do this - I mean, really, measuring how hot someone's tea is drunk? HARD - but this method has some clear elements of potential bias
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However, this bias may not be that big an issue Why is that?
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This one is pretty easy It's unlikely that the issues with tea heat measurement could've impacted group differences in cancer rates
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This means that errors in grouping would've (probably) occurred equally across groups, and if anything biased the results towards the null
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But there's another massive, glaring flaw that isn't touched on at all Reverse causality
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To think about this, let's rewind The hot tea/cancer hypothesis goes like this: hot tea = damage to throat = cancer
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But these people were measured at adulthood, and then followed over time The damage to their throat might have already happened So rather than the above sequence, we might see damage to throat = hot tea = cancer
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Is this likely? Well, the hot tea drinkers were also older, smoked more, used opium more often, less wealthy, and drank more at baselinepic.twitter.com/kZYIQGXxY4
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Given these confounders, is it likely that there was some reverse causality here?
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(Forgot to answer the question above btw, it was all of the above - the effect applies equally to groups, probably a minimal impact, and biases towards the null anyway)
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So, I'm just going to say it - yes, it's very hard in this situation to exclude reverse causality. There weren't any sensitivity analyses that really excluded it, so I think it's still pretty likely
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This raises the last massive issue - residual confounding Note how the authors describe their findings vs the Daily Mailpic.twitter.com/iCvBZCoW0N
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"Substantially strengthen the association" is NOT THE SAME AT ALL as "see their risk of gullet cancer rise by 90 percent" (Also, lol, gullet cancer is a great name)
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Why are the authors so cautious Maybe it's because TEA DRINKING IS SUPER SOCIAL AND HARD TO UNPICK FROM CONFOUNDERS It's in caps because GODDAM IT I SAY THIS EVERY WEEKpic.twitter.com/GUAQyTAYof
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We can't exclude residual confounding. Maybe people drink tea differently in different social situations. Maybe hot tea is more often served with alcohol. Maybe a single measure of how hot people drink their tea isn't that great a predictor 10 years later when they get cancer
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Maybe there are a million factors you can never measure, and never control for, and thus it's really hard to know if this is causal or not Maybe
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So the authors are cautious. They know that there's a decent chance that the heat of your tea has nothing to do with risk of esophageal cancer But the newspaper needs a story And so we get "hot tea causes cancer"pic.twitter.com/g6kancVrKs
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So, let's recap: Large epi study showed 0.34% absolute increase in risk of cancer associated with drinking the hottest tea compared to cold tea, in large amounts, every day for 10 years Reverse causality and residual confounding remain an issue
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Do we trust these results? Well, yes. The study was interesting, and well done Do we think hot tea = cancer?
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End of conversation
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