A new uncertainty methodology for t only nuclear engineering but also other disciplines is developed. This uncertainty methodology is analyzed by OECD/US-NRC Full Size Fine Mesh Bundle Tests (BFBT). Void distribution and critical power data for Boiling Water Reactor (BWR)bundles are utilized for the sensitivity and uncertainty analyses. Cumulative Distribution Functions for commonly used in two phase flow thermal hydraulic problems are developed and used as the inputs for the methodology. In addition Spanning Algorithm selects the limiting experimental cases to bound the experimental database. Finally, uncertainty methodology's feedback mechanism and flexibility provides a powerful tool for analyzers.