Due to the circumstances in some forensic cases, very small amounts of DNA (<100 pg) may be obtained. This, in turn, may affect the reliability of the PCR process, and so it may be advisable to repeat the amplification process for confirmatory purposes. Gill et al. [Forensic Sci. Int. 112 (2000) 17] proposed a method for the statistical evaluation of the PCR replicate information. In this paper we formalize the method proposed by Gill et al. [Forensic Sci. Int. 112 (2000) 17], and extend it to allow for cases involving mixed stains and for population substructure.