Result of recent fissile small sample worth experiments in three fast reactor diagnostic critical assemblies are presented. These experiments produced significant insight into heterogeneity-related errors in standard calculational models of worth experiments in plate-type critical assemblies. Results of improved techniques for calculation of worth experiments are presented, and the mean ratios of calculated to experimental worths in the three new benchmark critical assemblies are shown to be in the range from 0.97 to 1.04. The implications of the improved understanding of small sample worth experiments with respect to previously reported critical experiments are discussed.