A method of investigation a sample is provided, the sample being a mixture of DNA arising from more than one source. The method includes analysing the sample to obtain a genotype for the DNA present in the sample and assigning a prior probability distribution to the genotype. The likelihood function is considered and a posterior probability distribution for the genotype is established. In this way a probabilistic assessment of the genotype of the major or minor contributor to the sample can be obtained. This is beneficial over prior methods which use a deterministic method, and so involve the use of rule based methods.