Book excerpt: The Long Term Pavement Performance (LTPP) program has been collecting profile and International Roughness Index (IRI) information from more than 2,062 test sections since 1989 using K.J. Law 690DNC optical sensor Profilometers. Analysis of the IRI data has been limited, but with the increasing distribution of the LTPP DataPave software, this data is seeing increasing use. In an effort to confirm the quality of LTPP IRI data in the Information Management System (IMS) database and to document its variability, LTPP initiated an analysis of IRI variability in September 1997. This report documents the results of that study.