Not sure why you'd want to use sum. Average or median has got to be better. Sum simply increases the noise in proportion to signal. There is no rejection. Personally, I'd go with sigma reject but at a low rejection rate. I've found with six subs, you want a rejection rate of around 10%, the higher the sub set (say 12 to 16), the rejection rate is lower 4-5%. If you really want to use sum to do the work, decease the subs per set, so that the second pass will have a higher data rejection rate.
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