Dear Anette,
How different are your t-values between the revisions? I would expect
them to differ a little due to the altered way of avoiding
problematically low variance, but I would be a bit worried if they
were dramatically different. Basically, r4010 adjusted each t-contrast
standard error based on the contrast, while the latest revision
modifies the ResMS (variance estimate) image itself prior to any t- or
F-contrasts. The end result should be similar for t-contrasts.
Best,
Ged
On 13 April 2011 18:37, Vladimir Litvak <[log in to unmask]> wrote:
> Dear Anette,
> T-stats should be valid in all versions but they might slightly vary as
> there were some changes aimed at taking care of the low variance problem and
> other changes that GED and Guillaume know more about.
> Vladimir
> On 13 Apr 2011, at 14:32, Anette Giani <[log in to unmask]>
> wrote:
>
> Dear Vladimir,
>
>
>
> Thanks for your response.
>
> When redoing my statistics I realized that also my t-statistics changed.
> Could you explain why that is? Were t-statistics valid in 4010?
>
>
>
> Best,
>
> Anette
>
>
>
> From: Vladimir Litvak [mailto:[log in to unmask]]
> Sent: Tuesday, April 12, 2011 2:42 PM
> To: Anette Giani
> Cc: [log in to unmask]
> Subject: Re: F tests on M/EEG data
>
>
>
> Dear Anette,
>
>
>
> Yes F-tests were not valid in 4010. I'm surprised that you haven't seen it
> yourself because the results usually looked quite weird. This only pertains
> to SPM imaging source reconstructions results. Sensor level or beamformer
> images should be OK.
>
> Vladimir
>
> On 12 Apr 2011, at 12:49, Anette Giani <[log in to unmask]>
> wrote:
>
> Dear Vladimir,
>
> In the release note of the latest spm8-update it has been written:
>
> (ii) It is now enacted by modifying the residual mean squares image (ResMS)
> saved on disk. This permits F-contrasts for M/EEG data (and reduces leakage
> outside the brain in VBM F-contrasts with generous analysis masks). It also
> has the advantage that the tstatistics can be easily reproduced using the
> saved con_xxxx and ResMS images (though they still of course differ slightly
> to statistics recomputed from the original input images outside SPM).
>
>
>
> However, I do not exactly get what this means. Especially the sentences:
> “This permits F-contrasts for M/EEG data” confuses me. Does this imply that
> F-test on M/EEG data have been invalid for earlier spm8 releases? I am
> asking, since we are about to submit a paper and I am wondering if I have to
> redo statistics using the latest update? So far we calculated F-test on
> M/EEG data using SPM-r4010.
>
> Thanks in advance!
>
> LG,
>
> Anette
>
>
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