Comparison between the estimated SNR number from the SFR and objects identified in this paper. The lower limit of the identified object is the number of SNRs and the upper limit is the number in the parent sample including SNR candidates. The lower limit of the estimated SNR number is given by the optically visible time of SNRs for 10 000 yr and the upper limit corresponds to 20 000 yr. The dashed black line is the one-to-one relation. The shaded area provides coverage from two times the recovery rate to 50% of the recovery rate of SNRs.