Abstract: Due to the long implementation period of the electromagnetic compatibility (EMC) proficiency testing program, the uncertainty introduced by the long-term stability of the samples cannot be ignored, which will have a more significant impact on the calculation of the standard deviation of the proficiency assessment and thus on the evaluation of the results. This paper discusses the influence of the uncertainties introduced by the long-term stability of samples on the evaluation of the results of the proficiency testing program, and proposes that the uncertainties introduced by the long-term stability of samples should be taken into account when calculating the standard deviation of proficiency testing, so as to evaluate the results more objective and reasonable.