USRC Rating Definitions have been derived from technical publications originally developed by the Structural Engineers Association of Northern California (SEAONC). SEAONC granted the USRC permission to use these documents. Any differences between USRC Ratings and related SEAONC documents are solely the work of the USRC and do not reflect any opinion, endorsement, or approval by SEAONC. While US codes and engineering practices are among the most advanced in the world, evaluations of building performance subject to natural disasters include a significant amount of uncertainty: unknowns related to forecasting actual event location, size and duration, the actual intensity to which the building is subject, and the quality of the building design and construction. Current building analysis, evaluation and correlation methods do not address or remove all these sources of uncertainty. Due to many factors including but not limited to variations in construction, differing site conditions, and variations in natural and man-made events, the performance evaluation of a single building includes a significant amount of uncertainty.