A method and system for estimating the worst case corrosion in a pipeline for which pipeline wall thickness measurements are limited to sampled ultrasonic or radiography (UT/RT) measurements. A data library contains distributions of in-line inspection (IL) measurements for other pipelines, calibrated to correspond to UT/RT measurements as needed. These ILI datasets are randomly sampled multiple times, to obtain multiple sample sets from each ILI dataset. Candidate statistical distributions are evaluated for each sample set to determine which of the candidate statistical distributions most accurately estimates the worst case corrosion measured by ILI. A discriminant function is then derived from sample statistics and pipeline descriptors associated with the sample sets, along with the best candidate statistical distribution for that sample set. Sample statistics and pipeline descriptors for the pipeline with sampled UT/RT measurements are then applied to the discriminant function to determine the best one of the candidate statistical distributions for extreme value estimation, and the worst case corrosion is then determined using that best statistical distribution.