[Eeglablist] -log10(p) on ERP plots

[Arnaud Delorme]

p is indeed the p-value (not power — although it is power in other plots but in that case it is not -log10(power) but instead 10*log10(power) to scale power to dB).

For p-values, -log10(p) gives you the following

p=1 -> 0
p=0.1 -> 1
p=0.01 -> 2
p=0.001 -> 3

If we have log10(p), it would be -1, -2, -3, etc… so -log10(p) is better as it represents the number of decimals after the decimal point.

Arno

> On Jun 5, 2023, at 6:26 PM, Cedric Cannard via eeglablist <eeglablist at sccn.ucsd.edu> wrote:
> 
> Dear Denise,
> 
> The number of permutations should be printed in the command window when you run it, along with other information about the steps that EEGLAB is performing. You may need to scroll up in the command window to see them. 
> 
> Yes, time is on x-axis and normalized power is on the y-axis, "p" referees to power spectral density (PSD) here, not p-values. Significant differences will be displayed as a horizontal black bar on the time axis, because tests are run for each time point, and corrected with the FDR method here. Did you set p-value to 0.05 in the parameters as well, instead of "exact"?
> 
> The PSD is normalized (log10) to better account for interindividual differences (between electrodes and participants) and facilitate comparison of results across datasets. 
> 
> Hope this helps,
> 
> Cedric
> 
> 
> Sent with Proton Mail secure email.
> 
> ------- Original Message -------
> On Monday, June 5th, 2023 at 5:07 AM, Denise Dal-Lago via eeglablist <eeglablist at sccn.ucsd.edu> wrote:
> 
> 
>> To whom it may concern,
>> 
>> 
>> 
>> I am following the tutorial here: https://urldefense.com/v3/__https://eeglab.org/tutorials/10_Group_analysis/study_statistics.html*2-way-anova__;Iw!!Mih3wA!Am7k4qwUCPfiOtb-zclTnlZ1T0jzyYl2OFueIwN2PbU6LTIZhcYDKvC6nDqb5Huy5WLcN_mH_NtsZu2UscRW7FNCfMVY-A$ to run a 2x3 mixed ANOVAs using the GUI.
>> 
>> I need to run ‘permutation statistics’ with ‘FDR correction’. However, I am unsure about which permutation EEGLAB run by default. Where can I find this information?
>> 
>> 
>> 
>> Also, the ERP plots that I’ve obtained for the main effects and interaction
>> 
>> have "Time" on the X-Axis, and "-log10(p)" on the Y-Axis.
>> 
>> Shouldn't the Y-Axis simply indicate the "p-value"? Why do I have a negative log10(p) instead?
>> 
>> 
>> 
>> Many thanks for your help,
>> 
>> Denise
>> 
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