# Mark bad channels, interpolate and CAR: interpolate before or after CAR?

**URL:** <https://mne.discourse.group/t/mark-bad-channels-interpolate-and-car-interpolate-before-or-after-car/3962>\
**Category:** Support & Discussions\
**Tags:** preprocessing\
**Created:** [November 16, 2021, 10:21am UTC](https://mne.discourse.group/t/mark-bad-channels-interpolate-and-car-interpolate-before-or-after-car/3962 "2021-11-16T10:21:35Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![mscheltienne](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/mscheltienne/32/827_2.png) [@mscheltienne](https://mne.discourse.group/u/mscheltienne)\
**Post date:** [November 16, 2021, 10:21am UTC](https://mne.discourse.group/t/mark-bad-channels-interpolate-and-car-interpolate-before-or-after-car/3962/1 "2021-11-16T10:21:35Z")

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What is the best practice?

I have a dataset on which I am:

- marking the bad channels
- adding the reference channel
- filtering between (1., 45.) Hz
- computing a CAR reference and adding it as a projector (not applied)
- interpolating bad channels
- applying the CAR projector

Obviously, I want to compute CAR after marking the bad channels. But should I interpolate the bad channels before? Does it actually matter?

I am a bit confused, and I’m not sure what is happening to the projector when interpolating the bad channels.