Is it statistically valid to use the Grand Average transform to combine individual averages with different numbers of sweeps for each electrode (i.e. averaged in the individual channel mode)?
While keeping track of the (individual) n is the correct procedure, the question of whether using this option makes for “statistically valid” analyses has to be answered with “no”. The reason for this obviously is that different n’s will definitely lead to different signal-to-noise ratios between the channels in your setup and thus will automatically lead to different standard deviations due to the increased amount of residual spontaneous EEG activity left in the resulting average for lower-n channels. The reason this option was included in BrainVision Analyzer is because we have a lot of customers working with clinical populations such as schizophrenics, that are very difficult to work with, and tend to produce enormous amounts of potential artifacts. For this type of population, our customers are glad for any trial/channel combination they can use for the average, regardless of whether or not this means making compromises regarding the validity of subsequent statistical comparisons.
