Using Workbench Command

PERFORM REDUCTION OPERATION ACROSS SUBVOLUMES
   wb_command -volume-reduce
      <volume-in> - the volume file to reduce
      <operation> - the reduction operator to use
      <volume-out> - output - the output volume

      [-exclude-outliers] - exclude non-numeric values and outliers by standard
         deviation
         <sigma-below> - number of standard deviations below the mean to
            include
         <sigma-above> - number of standard deviations above the mean to
            include

      [-only-numeric] - exclude non-numeric values

      For each voxel, takes the data across subvolumes as a vector, and
      performs the specified reduction on it, putting the result into the
      single output volume at that voxel.  The reduction operators are as
      follows:

      MAX: the maximum value
      MIN: the minimum value
      INDEXMAX: the 1-based index of the maximum value
      INDEXMIN: the 1-based index of the minimum value
      SUM: add all values
      PRODUCT: multiply all values
      MEAN: the mean of the data
      STDEV: the standard deviation (N denominator)
      SAMPSTDEV: the sample standard deviation (N-1 denominator)
      VARIANCE: the variance of the data
      TSNR: mean divided by sample standard deviation (N-1 denominator)
      COV: sample standard deviation (N-1 denominator) divided by mean
      L2NORM: square root of sum of squares
      MEDIAN: the median of the data
      MODE: the mode of the data
      COUNT_NONZERO: the number of nonzero elements in the data