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The Master Curve method and the associated reference temperature, asdefined in the new test standard ASTM E1921, is rapidly moving from the researchlaboratory to applications in the U.S. commercial nuclear power industry. The Toreference temperature is very robust, and it has rapidly become a tool to investigatematerial inhomogeneity effects and constraint effects because it is capable of measuringdifferences imperceptible using earlier methods that have been used to define the ductile-tobrittletransition in structural ferritic steels. Previous work by the present authors[1-2] hasshown that To is sensitive to the specimen geometry used in its evaluation. While thisgeometry sensitivity is expected if shallow crack specimens are compared with deep crackspecimens, a significant difference also appears to exist between deep crack C(T) andSE(B) geometries. This difference causes concern in the application of ASTM E1921 toregulatory decisions facing the US Nuclear Regulatory Commission. In this work a study ismade to separate the effects of material inhomogeneity and specimen geometry for the sameA533B pressure vessel material studied extensively in the previous work. The results showthat the To differences due to material inhomogeneity can be separated from that due tospecimen geometry, at least if the data set utilized is extensive enough.