and delayed signals are required to be 0.7-12 MeV and 6-12 MeV, respectively. The temporal separation between a pair of prompt and delayed signals should be within 1-200 $\mu$s. To reject cosmogenic backgrounds, the delayed signal is required to be 600 $\mu$s, 1000 $\mu$s, or 1 s later than a muon, depending on the deposited energy of the muon. Finally, no other signal should occur in 200 $\mu$s before the prompt signal and after the delayed signal. Two independent algorithms are developed following these criteria with minor differences. The selected samples differ by less than 10%, mostly due to the different energy calibration used.

A detailed treatment of the absolute and relative efficiencies was reported in Refs. [@ad12; @DB_CPC]. The uncertainties of the absolute efficiencies are correlated among the ADs and are thus negligible in oscillation analyses. The determination of all relative uncertainties is data-driven. The dominant ones come from the energy calibration and the neutron capture fraction on Gd, both at the 0.1% level. The total relative uncertainty is conservatively estimated to be 0.2%, uncorrelated among ADs.

Five kinds of background are considered in Daya Bay. The accidental background is the largest one but contributes negligible to the total 