[en] The desire for improved nuclear material safeguards has led to the development and use of a number of techniques and instruments for the non-destructive assay (NDA) of special nuclear material. This paper discusses sources of potential bias in NDA measurements and suggests methods of eliminating the effects of bias in assay results. Examples are given of instruments in which these methods have been successfully applied. The results of careful attention to potential sources of assay bias are a significant reduction in the number and complexity of standards required for valid instrument calibration and more credible assay results. (author)