A variation on the coincidence counting technique described above. Coincidence counting involves measurement of the total number of neutrons detected (singles rate) and a statistical determination of the number of 2-fold coincidences (doubles rate) by analyzing the time history of the detected neutrons. The measurement of large Pu or U samples with neutron coincidence counting requires additional assumptions and mathematical analysis to account for neutron multiplication and to accurately determine nuclear material mass. Multiplicity counting includes circuitry to determine higher order coincidences (e.g. triples rate); this allows the direct measurement of neutron multiplication without additional assumptions. This technique is useful to measure impure nuclear materials where the assumptions required for 2-fold coincidence counting are not met. Multiplicity counters usually have a very high efficiency (>60%) because this is needed to measure 3-fold, or higher order, coincidences with reasonable counting times.