The analysis of counter performance by cusum techniques

It is necessary in most radiochemical studies to check periodically the efficiency of the detector with a standard or reference source to ensure its constancy. This can lead to a large number of readings which can best be analysed graphically by means of cumulative summation. Minor abrupt changes in counter performance can often be determined by retrospective examination of cusums. Alternatively in an on-going situation the reference source readings may be plotted as they are taken and a significant change detected comparatively rapidly with a V-mask. The standard deviation used in tests of significant change has in practice been found to be greater than that expected simply from the randomness of the disintegration process. The point of change can often be located precisely so that it should be possible if necessary to correct other measurements with the detector.