<P> In statistical surveys , when subpopulations within an overall population vary , it is advantageous to sample each subpopulation ( stratum ) independently . Stratification is the process of dividing members of the population into homogeneous subgroups before sampling . The strata should be mutually exclusive : every element in the population must be assigned to only one stratum . The strata should also be collectively exhaustive : no population element can be excluded . Then simple random sampling or systematic sampling is applied within each stratum . The objective is to improve the precision of the sample by reducing sampling error . It can produce a weighted mean that has less variability than the arithmetic mean of a simple random sample of the population . </P>
<P> In statistical surveys , when subpopulations within an overall population vary , it is advantageous to sample each subpopulation ( stratum ) independently . Stratification is the process of dividing members of the population into homogeneous subgroups before sampling . The strata should be mutually exclusive : every element in the population must be assigned to only one stratum . The strata should also be collectively exhaustive : no population element can be excluded . Then simple random sampling or systematic sampling is applied within each stratum . The objective is to improve the precision of the sample by reducing sampling error . It can produce a weighted mean that has less variability than the arithmetic mean of a simple random sample of the population . </P>
when should a researcher use stratified random sampling
when should a researcher use stratified random sampling
when subpopulations within an overall population vary
when subpopulations within an overall population vary