How do I calculate the signal to noise ratio of a signal without knowing the noise of the signal?

I was given an EEG channel and asked to epoch the data around given stimulus times. I plotted the average of these epochs AFTER rejecting any epoch that contained an EEG value greater or less than +/- 300 μV. I am now being asked what the signal to noise ratio (SNR) of this average is? I don't understand this because I am not sure what the noise is in this signal? The videos I have watched seem to need you to know what the noise is to calculate SNR. And the snr function requires you know the noise too. What would be the noise in this signal and how would I calculate this?

Answer 1

[Jeff Miller]

I'm not sure whether this fits your situation, but one way to estimate the noise is to subtract the average of the epochs from each individual epoch. The assumption would be that the real signal is what you see in the average and that the noise is whatever is left over in the individual epochs after you removed that signal.