During the diagnosis of existing structures, errors can be generated by uncertainty of measurements. The latter are affected by several factors and should be evaluated on-site. In this paper, a large data of resistivity values (1800 measurements) measured on a bridge in a marine environment has been gathered utilizing a four electrodes probe under specific procedures. The uncertainty assessment based on measurements repeatability or local material anisotropy and variability were characterized from this database. Moreover, considering the uncertainty results together with the corrosion risk from RILEM TC154 recommendation, Probability of Wrong Assessment (PWA) is evaluated. Finally, the influence of uncertainty and PWA were discussed in order to help the owner of the bridge in taking decision on additional measurements or complementary investigations.