1. A process for determining similarities of signals, comprising: receiving first and second signals related to material composition, wherein the first and second signals are derived from analyses of two similar samples or an analysis of a single sample; comparing, by operation of a processor, a first characteristic element of the first signal with a second characteristic element of the second signal to generate a ratio, wherein the first and second characteristic elements are homologous characteristic elements; determining values for parameters of an uncertainty model using the ratio, a statistical variable based on the ratio, or both; determining a non-zero composition variation threshold, above which two samples are considered to be derived from two separate sources, using the uncertainty model; and determining a similarity measurement between at least two other signals derived from two samples, based on each parameter value of the uncertainty model and of homologous characteristic elements in the at least two other signals whose similarity is being measured; and comparing the similarity measurement with the composition variation threshold to determine whether the at least two other samples are from interconnected or separated sources.