True coincidence summing (TCS) corrections almost always need to be applied in gamma-ray spectrometry whenever a radionuclide is present in the sample with a complex decay scheme. Namely, in such a case more than one gamma-ray can be emitted by the nucleus at the same time and simultaneously detected, which leads to the loss of counts from the corresponding peak areas of the gamma lines involved when compared to peak areas of equivalent mono-energetic emitters. Since the full-energy peak efficiency, the knowledge of which is required for the computation of the activity of a given radio-nuclide based on its full-energy peak areas, is defined for mono-gamma emitters, the summing effect needs to be corrected for, both in the calibration and in the analysis phase of the measurement.
The TCS correction factors can be measured, but the preparation of the required standards is involved and costly, so computational methods are increasingly being used instead. These are always based on a detector model that never exactly corresponds to the real detector, which introduces an error in the calculation of the TCS correction factors. More specifically, one can speak of the error in the computation of the full-energy peak and total efficiencies, which serve as input to the calculation of the TCS correction factors themselves.