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So, what did the scientists do? Well, on first blush it looks like they did a simple cohort study - took a group of people, split them up by whether they were exposed to cats/dogs, and compared their mental health years laterpic.twitter.com/QzofRQIDQm
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But then you look a little closer, and you realize that this is actually a CASE-CONTROL study, because the authors selected their patients based on outcomes (mental health diagnosis), not exposure (having pets)pic.twitter.com/DSqVa6SVLF
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(Tip for non-epi people: the difference between a case-control and cohort can be hard to get at first, but just look at how they selected their participants. If they picked people based on their outcomes, it's case-control)
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Ok, so it's a case-control being reported as a cohort study That's a problem, maybe, but not that much Let's look at the resultspic.twitter.com/0UWqXnAYrn
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And this is where things get really, REALLY weird Compare this sentence in the results to the graph. Notice anything strange here?pic.twitter.com/gPsfY11Oa6
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(Note: I'm not talking about how they used cross-sectional data to fudge longitudinal analyses. We'll get to that)
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The sentence I highlighted above is just...wrong If that's the analysis they did, then they didn't show that "the time of the first household pet dog...was associated with a significantly decreased hazard of having a schizophrenia diagnosis"
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This graph shows a Kaplan-Meier curve plotting the proportion who got a dog against the age that they reported getting one, by group It's testing whether people who are currently diagnosed with SP or BPD were more likely to get a dog as a child!pic.twitter.com/8TpWFUdsz3
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In other words, the main finding of this paper, the one reported everywhere, is NOT that dogs reduce your risk of schizophrenia, it's that schizophrenia reduces your risk of dogs!pic.twitter.com/F3Bd14tNLP
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Now, it's not the fault of the media that this was reported wrong - it's wrong IN THE PAPER This is perhaps not surprising, because there are quite a few other mistakes...
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This sentence is statistically weird. If you're correcting for multiple comparisons (good) you can't just undo that by using the 'trend'pic.twitter.com/AiaTknBffm
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The authors also report 'relative risks' even though they used logistic models You ~can~ convert the results of a logistic model to a relative risk, but it's not that easy and they didn't report doing so, which means the RRs are probably just ORspic.twitter.com/OB1QLZ7jJA
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But all of this pales in comparison to the massive issue I skipped past earlier The study used longitudinal analysis (Cox/KM) to analyze cross-sectional datahttps://twitter.com/GidMK/status/1220462617472487425?s=20 …
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You'd think from the results that they had monitored children over time, recorded when they got a pet, and then followed up years later to find out if they got a diagnosis But remember - this is a CASE-CONTROL study
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What the authors actually did is ASK PEOPLE WITH A CURRENT DIAGNOSIS when they remembered getting a dog as a child This is an inherently flawed approachpic.twitter.com/Kwo4i0guDD
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Imagine asking someone who is currently going through an inpatient admission for acute psychosis when they remember getting a dog as a child, and using that as your exposure variable You see the problem
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So this study didn't even really measure whether people who were diagnosed with schizophrenia were more likely to get dogs as children It measured who was more likely to REMEMBER getting a dog as a childpic.twitter.com/9wIOr6YxN0
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And using the time that they remembered having a dog as the time-point for a longitudinal analysis is...problematic For one thing, you don't have accurate pet death data. You don't actually know how long these people were exposed to pets!
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I'm going to stop here, because while there's more there really isn't much point in going through it allpic.twitter.com/9ZPrh2WLfs
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The fact that the analyses were the wrong way around, the exposure was a bit meaningless, and that the stats were probably misreported is probably enough And yet, the study was published, and got into the NYT *sigh*
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TL:DR - dogs don't prevent schizophrenia - schizophrenia may prevent dogs - even then, probably not - this study is a mess
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If you want more info,
@statsepi and@ADAlthousePhD explain really well in this thread:https://twitter.com/statsepi/status/1220015078726098944?s=20 …Show this thread
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