All the experiments included in the PDG average were finished more than a decade ago. The essential features of the experiments are very similar and the approach was kept quite simple. Naturally, beam stopping and detection techniques varied among the experiments listed in the table but several systematic issues were common to all. A residual polarization in the stopped muons presents a very serious potential problem.
Muon precession can change with time the angular distribution of decay positrons.
Any non-uniformities in the detector system will then lead to a direct timing error. Various techniques have been employed to account for the this potentially serious problem. Another systematic issue, particularly for higher-rate experiments, is pileup, when two decay positrons enter the same detector element within a short period of time and are registered as one. Because the probability for this to occur is larger at the beginning of the measuring interval compared to at the end, it is a potentially serious source of systematic error. The effect of pileup can be mitigated by increasing detector segmentation, limiting the instantaneous rate, keeping phototube signals short and improving the time resolution of the readout electronics.