# relating dSPM to p

**URL:** <https://mne.discourse.group/t/relating-dspm-to-p/294>\
**Category:** Mailing List Archive (read-only)\
**Tags:** list-archive\
**Created:** [April 11, 2011, 5:53pm UTC](https://mne.discourse.group/t/relating-dspm-to-p/294 "2011-04-11T17:53:27Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![system](https://global.discourse-cdn.com/free1/uploads/mne/original/1X/85cc6bd2b69cb698a166dc6d880fb550510d0144.jpeg) [@system](https://mne.discourse.group/u/system)\
**Post date:** [April 11, 2011, 5:53pm UTC](https://mne.discourse.group/t/relating-dspm-to-p/294/1 "2011-04-11T17:53:27Z")

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Hi All,

I am interested in relating the grandaveraged dSPM values for a set of  
subjects to a particular p-value. I know this has come up a few times on  
both this and the old mne\_analysis listserv, but I am confused about a few  
issues in particular:

First, I have seen several users mention using the function inv\_fpdf. As I  
understand it, this function relates a p-value to dSPM by treating dSPM as  
an f statistic with degrees of freedom (3\* number of subjects) / (number of  
time points used to compute the noise covariance matrix). How can I grab  
the number of time points in the noise covariance matrix? I think  
?mne\_show\_fiff [file] --tag 3531 ?verbose? (where [file] is the covariance  
matrix for one of my subjects) may return this value, but I?m not completely  
clear on this even after looking at the help.

Also, to correct for multiple comparisons, I was considering doing a  
bonferroni correction with the correction factor equal to (n timepoints) x  
(# vertices). However, I was told this isn?t a good idea because 1) results  
are correlated across both time and space and 2) the forward solution is  
only calculated for a max of 20,000 vertices (not every single one). Can  
anyone recommend a better correction?

Finally, in addition to finding the threshold for the grandaverage of  
conditions, I?d also like to visualize grandaveraged difference waves.  
Should these be treated any different statistically?

Any help would be greatly appreciated!

Thanks,

Matt  
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