[en] Japan Atomic Energy Agency (JAEA) and Los Alamos National Laboratory (LANL) jointly developed the Epithermal Neutron Multiplicity Counter (ENMC). A measurement test was performed using the standard samples and its results showed that ENMC achieves high measurement accuracy (approx. 0.4%) under the optimum conditions which is the same level with destructive assay. However, in the practical measurement for nuclear material accountancy or safeguards, bias is observed due to the variation of the sample properties. With this recognition, JAEA jointly with LANL conducted simulations for identifying the causes of this bias and its correction method. The results showed that the dominant cause of the bias is difference in sample density and this bias can be mitigated by correcting neutron counting efficiency. JAEA and LANL evaluated the applicability of the two ways for correcting the counting efficiency; either applying simulation data or real measurement data. It was concluded that as application of real measurement data has certain difficulties, the simulation data is more reasonable to be applied for the correction of the counting efficiency. (author)