# confusion about the results of combine evoke

**URL:** <https://mne.discourse.group/t/confusion-about-the-results-of-combine-evoke/3706>\
**Category:** Support & Discussions\
**Tags:** eeg, epochs\
**Created:** [September 16, 2021, 12:22pm UTC](https://mne.discourse.group/t/confusion-about-the-results-of-combine-evoke/3706 "2021-09-16T12:22:17Z")\
**Posts on this page:** 1\
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**Author:** ![mmagnuski](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/mmagnuski/32/153_2.png) [@mmagnuski](https://mne.discourse.group/u/mmagnuski)\
**Post date:** [September 17, 2021, 11:34am UTC](https://mne.discourse.group/t/confusion-about-the-results-of-combine-evoke/3706/4 "2021-09-17T11:34:56Z")

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Good point @richard. So, to extend my previous answer - as stated in the code comment the `.nave` parameter is used as an estimate of variance (`1 / nave`) during inverse operations (projecting data to source space). The variance accumulates when you perform difference operations (subtracting two noisy signals gives you even noiser difference) so the `.nave` changes to reflect that - and as a result is lower than your original `.nave`.

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