Washington River Protection Solutions (WRPS) under contract from the U.S. Department of Energy (DOE) is responsible for assessing the condition of the double-shell tanks (DST) on the Hanford nuclear site. WRPS has contracted with AREVA Federal Services LLC (AFS) to perform ultrasonic testing (UT) inspections of the 28 DSTs to assess the condition of the tanks, judge the effects of past corrosion control practices, and satisfy a regulatory requirement to periodically assess the integrity of the tanks. Since measurement inception in 1997, nine waste tanks have been examined twice (at the time of this report) providing UT data that can now be compared over specific areas. During initial reviews of these two comparable data sets, average UT wall-thickness measurement reductions were noted in most of the tanks. This variation could be a result of actual wall thinning occurring on the waste-tanks walls, or some other unexplained anomaly resulting from measurement error due to causes such as the then-current measurement procedures, operator setup, or equipment differences. WRPS contracted with the Pacific Northwest National Laboratory (PNNL) to assist in understanding why this variation exists and where it stems from.