which can be made a minimum if the sampling rate within each stratum is made proportional to the standard deviation within each stratum: n h / N h = k S h {\displaystyle n_{h}/N_{h}=kS_{h}} , where S h = V a r h {\displaystyle S_{h}={\sqrt {Var_{h}}}} and k {\displaystyle k} is a constant such that ∑ n h = n {\displaystyle \sum {n_{h}}=n} .