The invention provides methods for providing information on the probability of DNA samples being contaminated over days, weeks and months in relation to collection, processing and analysis of DNA samples. Methods are also provided for analysing the likelihood of a result arising due to contamination and/or determining the analysis protocol to be applied to a DNA sample and/or methods of operating databases, particularly containing DNA profiles.Improvements in accounting for sporadic and undetected contamination and improvements in the operation of DNA sampling of analysis are provided by the invention.