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By Daniel Bor in neuroimaging Over this week, there has been a striking debate in the blogosphere and on Twitter concerning the flaws in many published neuroimaging studies. His partial explanation was that this was in a different age, with more lax conventions and admittedly he was only a minor author on the paper himself.
Late Tuesday night, Neurocritic posted a provocative blog article in response to this, asking the question: Two key issues quickly surfaced: I thought it might help in this discussion to explain one of the main statistical issues that this debate is pinned on, that of corrected versus uncorrected statistics, and how this applies to brain-scanning.
And if many published imaging papers are so flawed, I want to try to explain how the literature became so sloppy. Just to flag up that this blog is addressing two audiences.
I wanted to explain the context of the debate to a general audience, which occurs in the next two sections, and suggest how they can assess neuroimaging stories in the light of this in the last small section.
The middle sections, although hopefully understandable and maybe even of some interest to all, is directed more at fellow scientists. So what are corrected and uncorrected statistics?
Imagine that you are running some experiment, say, to see if corporate bankers have lower empathy than the normal population, by giving them and a control group an empathy questionnaire. How can you tell whether this is just some random result, or that bankers really do have lower empathy?
This is the point where statistical testing enters the frame.
Classically, a statistical test will churn out a probability that you would have got the same result, just by chance. All well and good, but what if you also tested your control group against politicians, estate agents, CEOs and so on? His mojo must be building! So he tries again, and again and again.
Then, as if by magic, on the 20th attempt, he gets all 4 heads. Joe Superstitious proudly concludes that he is in fact very skilled at telekinesis, puts the coin in his pocket and saunters off.
Joe Superstitious was obviously flawed in his thinking, but the reason is actually because he was using uncorrected statistics, just as the empathy study would have been if it concluded that bankers are less empathic than normal people.
If you do multiple tests, you normally have to apply some mathematical correction to take account of how many tests you ran. How does this apply to brainscanning? Moving on to neuroimaging, the data is far more complex and inordinately larger, but in essence exactly the same very common statistical test one might have used for the empathy study, a t-test, is also used here in the vast majority of studies.
So there is a vast problem of some of these voxels to be classed as significantly active, just by chance, unless you are careful to apply some kind of correction for the number of tests you ran.
This is still in relatively common use today, but it has been shown, many times, to be an invalid attempt at solving the problem of just how many tests are run on each brain-scan. Poldrack himself recently highlighted this issue by showing a beautiful relationship between a brain region and some variable using this threshold, even though the variable was entirely made up.
In a hilarious earlier version of the same point, Craig Bennett and colleagues fMRI scanned a dead salmonwith a task involving the detection of the emotional state of a series of photos of people.
So the take home message is that we clearly need to be applying effective corrections for the large quantities of statistical test we run for each and every brain activation map produced. But in almost all other circumstances, we should all be using corrected significance, and reviewers should be insisting on it.
Should we retract uncorrected neuroimaging papers? Surprisingly, there is a vast quantity of published neuroimaging papers, even including some in press, which use uncorrected statistics.Type or paste a DOI name into the text box.
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|For Reviewers||This was true when I started this article, but no longer. I bought my '88 K75S on October 14,|
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The Psychopathy Checklist or Hare Psychopathy Checklist-Revised, now the Psychopathy Checklist—revised (PCL-R), is a psychological assessment tool most commonly used to assess the presence of psychopathy in individuals. It is a item inventory of perceived personality traits and recorded behaviors, intended to be .
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