Hi,
Yes - the same applies at first and higher levels.
All the best,
Mark
On 10 Jan 2009, at 04:40, Shih-Wei Wu wrote:
> Thanks very much for the info.
> One quick follow-up question: For experiments that will focus on
> analyzing individual subject's data, like a psychophysics experiment
> that will run perhaps 6 subjects and analyze each subject's data
> separately, does the same criterion (using cluster thresh with
> z=2.3, p=0.5) apply?
>
> thanks
> shihwei
>
>
>
> On Thu, Jan 8, 2009 at 3:46 AM, Mark Jenkinson <[log in to unmask]>
> wrote:
> Hi,
>
> Our default settings (z=2.3 and p=0.05) is perfectly acceptable and
> generally
> fairly useful - that's why it is the default!
>
> There is no simple relation between uncorrected and corrected p-
> values.
> The relation is given by Gaussian Random Field Theory but it depends
> mainly
> on the size of the cluster formed after the initial z threshold is
> applied. The
> actual maths of how it converts this z threshold value and cluster
> size into
> a corrected p-value is at the heart of the theory and you should
> read the relevant
> papers if you want to understand this. Note that we have a
> technical report on
> our website which summarises some of the early papers on this.
> There is also
> an excellent webpage discussing this and many issues at
> http://imaging.mrc-cbu.cam.ac.uk/imaging/PrinciplesRandomFields
>
> As for lowering the z threshold, it is an arbitrary value and as
> long as the threshold
> is high enough, as otherwise the approximations in the theory break
> down.
> So we do not recommend lowering it much below 2.3. Your value of
> 1.6 is
> really too low for the inference to be accurate. In this case we
> would recommend
> you try other methods of inference such as the ones that randomise
> offers.
> This includes the new Threshold-Free Cluster Enhancement (TFCE) method
> which neatly side-steps the problem of having to specify any
> threshold.
> See http://www.fmrib.ox.ac.uk/fsl/randomise/index.html for more
> details on
> randomise.
>
> All the best,
> Mark
>
>
>
>
> On 7 Jan 2009, at 22:26, shih-Wei Wu wrote:
>
> Dear all,
>
> This is a rather arbitrary question, but I am curious if there is a
> general, accepted criteria
> for setting up cluster z threshold and p threshold for whole-brain
> analysis at the group-level
> (mixed effects FLAME 1). Would the default setting (z=2.3, p=.05) be
> good enough?
>
> A second question is how I should interpret activation when I lower
> cluster threshold to,
> say, z=1.6, but keep p=0.05. In this case, I do get clear
> activations at areas that conform
> to my hypothesis, and it doesn't look like there are cluster
> activations all over the brain. In
> my particular case, there is no cluster in the brain that satisfies
> the default criterion. And
> that is why I lowered the z thresh.
>
> The last question is what is the relation between 'cluster
> threshold' and 'uncorrected'? For
> example, if a paper reports activation p<0.001 (uncorrected), what
> would that normally
> correspond to when using 'cluster threshold'?
>
> thanks
> shihwei
>
>
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