So we could be talking about a trial where the authors simply reviewed medical records or a randomized trial where they intervened I'm personally currently involved in both, and everything in between
Another potential source of bias: they allowed patients to self-select into treatment groupspic.twitter.com/vPI2Iah9Tw
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The answer to both polls is, of course, all of these things. Most potential sources of bias have many causes and indeed many impacts
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Both of these sources of bias would likely make the treatment group appear better - if they did have an impact - and so reduce the strength of the study's results
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HOWEVER it's important to remember that this is not true for all sources of bias What you need to look for is whether the bias applies equally to all groups, or unequally to one or more groups in the study
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What do I mean by this? Well, here's an example from the recent study on carbs and health in the Lancet: reporting biaspic.twitter.com/ef2Q4Kw7Hg
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We know that people under-report their food intake to medical professionals This is not surprising. No one likes to admit to their doctor that they aren't following the diet they're supposed topic.twitter.com/7MMFIzIlGe
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But, AND THIS IS IMPORTANT, this reporting bias impacts every single person in the study in a similar way Thus, it shouldn't impact the results, except to potentially make them seem LESS meaningful than they actually are
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If someone is bashing a piece of research, take a look at their arguments and think: "Is this really going to influence the results?" "Will it impact all groups equally?" "WHAT impact will it have?" These are all vital questions to answer
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They are also important for you yourself if you want to know more about the research and its usefulness
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This has been my brief thread on bias. If anyone's interested, let me know and I'll do another on different sources of bias or expand on this one
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End of conversation
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