  In a systematic search of the OTs at GRBs the Swift satellite determined only
an upper limit of the apparent brightness in a significant fraction of cases.
Combining these upper limits with the really measured OT brightness we obtained
a sample well suited to survival analysis. Performing a Kaplan-Meier product
limit estimation we obtained an unbiased cumulative distribution of the V
visual brightness. The lg(N(V)) logarithmic cumulative distribution can be well
fitted with a linear function of V in the form of lg(N(V))= 0.234 V + const. We
studied the dependence of V on the gamma ray properties of the bursts. We
tested the dependence on the fluence, T90 duration and peak flux. We found a
dependence on the peak flux on the 99.7% significance level.
