The concept is somewhat nebulous, and statistics are involved.[4] A group of America researchers in 2018 posed themselves the question how similar must a product be in order to be well-represented by the tested reference sample?[5] Because the concept was derived from the EPA, chemical similarity and biological similarity are equally important.[5] The concept is employed "so that safety data from the tested reference can be applied to untested materials,"[5] because "when toxicity data are not available for a chemical mixture of concern, US EPA guidelines allow risk assessment to be based on data for a surrogate mixture considered “sufficiently similar” in terms of chemical composition and component proportions."[1]