The proportionator is the most efficient unbiased stereological method used to estimate population size in samples. A typical application is counting the number of cells in an organ. The proportionator is related to the optical fractionator and physical dissector methods that also estimate population. The optical and physical fractionators use a sampling method called systematic uniform random sampling, or SURS. Unlike these two methods the proportionator introduces sampling with probability proportional to size, or PPS. With SURS all sampling sites are equal. With PPS sites are not sampled with the same probability. The reason for using PPS is to improve the efficiency of the estimation process.