Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help ft_sourcegrandaverage”.

  FT_SOURCEGRANDAVERAGE averages source reconstructions over either multiple
  subjects or conditions. It computes the average and variance for all
  known source parameters. The output can be used in FT_SOURCESTATISTICS
  with the method 'parametric'.
  Alternatively, it can construct an average for multiple input source
  reconstructions in two conditions after randomly reassigning the
  input data over the two conditions. The output then can be used in
  FT_SOURCESTATISTICS with the method 'randomization' or 'randcluster'.
  The input source structures should be spatially alligned to each other
  and should have the same positions for the source grid.
  Use as
   [grandavg] = ft_sourcegrandaverage(cfg, source1, source2, ...)
  where the source structures are obtained from FT_SOURCEANALYSIS or
  from FT_VOLUMENORMALISE, and the configuration can contain the
  following fields:
    cfg.parameter          = string, describing the functional data to be processed, e.g. 'pow', 'nai' or 'coh'
    cfg.keepindividual     = 'no' or 'yes'
  To facilitate data-handling and distributed computing you can use
    cfg.inputfile   =  ...
    cfg.outputfile  =  ...
  If you specify one of these (or both) the input data will be read from a *.mat
  file on disk and/or the output data will be written to a *.mat file. These mat
  files should contain only a single variable, corresponding with the
  input/output structure. For this particular function, the input data
  should be structured as a single cell array.