15.8.2  AVERAGE 

This  option  will  not  work  for  the  SUBTRACT  command.  If AVERAGE is
specified, the data is averaged together according to  the  Y  statistics.
If  Y  is non zero but DY is 0 then DY is assumed to be 1.  If both Y1 and
DY1 are zero then Y3,DY3 = Y2,DY2.
     Y3=(Y1/DY1**2+Y2/DY2**2)/(1/DY1**2+1/DY2**2)
     DY3=SQRT(2*DY1**2*DY2**2/(DY1**2+DY2**2))
If you wish to average several data sets with statistics, this should give 
you the correct result.  Zero data  with  zero  statistics  indicates  the
absence  of  data.   If  you  wish  to  average  several data sets without
statistics then you should always set the  statistics  to  a  "reasonable"
value.