GNhs_m.txt (drop last 4 columns)
Monthly mean group number from the original series by Hoyt & Schatten 1998.
(-1 values indicate "no data")
Col 1: Year
Col 2: Month
Col 3: Date in decimal year (middle of month)
Col 4: Group number Gn
Col 5: Standard error Eg
Col 6: Number of observations