Techniques to determine if an individual is a member of an ensemble contributing to combined genomic data include determining a site frequency spectrum (SFS) for alleles in the combined genomic data, and a number N of individuals contributing to the combined genomic data. Based on the SFS and N, both a number of queries NQ and a number of yeses NY are determined. NY is a number of yes responses to NQ queries, which indicates a particular likelihood that the individual is a member of the ensemble. Output is based on at least one of NQ and NY. For example, fewer than NQ queries from a single source are answered from a beacon. Alternatively, the likelihood that an individual is a member of the ensemble is determined based on responses to NQ queries about NQ different alleles present in the individual and NY.