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Prescreening is a methodology where forensic examiners select samples similar to given trace evidence to represent the background population. This background evidence helps assign a value of evidence using a likelihood ratio or Bayes factor. A key advantage of prescreening is its ability to mitigate effects from subpopulation structures within the alternative source population by isolating the relevant subpopulation. This paper examines the impact of prescreening before assigning evidence value. Extensive simulations with synthetic and real data, including trace element and fingerprint score examples, were conducted. The findings indicate that prescreening can provide an accurate evidence value in cases of subpopulation structures but may also yield more extreme or dampened evidence values within specific subpopulations. The study suggests that prescreening is beneficial for presenting evidence relative to the subpopulation of interest, provided the prescreening method and level are transparently reported alongside the evidence value.