Asset management requires the development of performance indicators (PIs) and decision procedures. Within the French RERAU methodology each rehabilitation criterion is assigned a grade out of four possible ones. This grade results from an aggregation of complementary PIs that use information derived from various sources: visual inspection, O&M data, network monitoring, etc. This paper focuses on the development of dysfunction indicators derived from visual inspection results (WP1 of the French INDIGAU program). Inspection reports provide sequences of observation (defect) codes. On this basis, three complementary procedures are proposed so as to assign a condition grade to the sewer segment: (a) expert rules identifying major defects (b) calculation of a single score and comparison to three thresholds and (c) rules based on the analysis of scores distribution along the segment. Calibrating procedure b) means defining parameters used in the calculation of a single score and defining three (crisp or fuzzy) thresholds. The calibration also requires experts' judgments that will be used as references. A sample of 45 links has been studied by 8 experts, regarding 10 indicators. The results display a lot of conflicts between experts' opinions. Three types of situation are defined: 1) no conflict, a consensus can be identified; 2) one expert disagrees with a majority, a consensus may be defined; 3) major conflicts between answers.