A study is planned to estimate the mean concentration of a pollutant in stream sediments in a defined area at a given point in time. The sediments have been samples extensively in past years in this tame area, so the variability in the measurements is known to be 2 = 100 (ppm)2. We assume 2 /N is ~ 0, that is N is very large relative to . Study objectives require that enough measurements be taken such that Var (x) will be no larger than V = 4 ppm2