Stratified Random Sampling
Stratified random sampling refers to a sampling method that has
the following properties.
In this tutorial, we will assume that the researcher draws a
simple random sample from each stratum.
Advantages and Disadvantages
Stratified sampling offers several advantages over simple random sampling.
We can ensure that we obtain sufficient sample points to support a separate
analysis of any subgroup.
The main disadvantage of a stratified sample is that it may require more
administrative effort than a simple random sample.
Proportionate Versus Disproportionate Statification
All stratified sampling designs fall into one of two categories, each of which
has strengths and weaknesses as described below.
Proportionate stratification. With proportionate
stratification, the sample size of each stratum is proportionate to the
population size of the stratum. This means that each stratum has the same
Proportionate stratification provides equal or better precision
than a simple random sample of the same size.
Gains in precision are greatest when when values within strata are
Gains in precision accrue to all survey measures.
Disproportionate stratification. With disproportionate
stratification, the sampling fraction may vary from one stratum to the next.
The precision of the design may be very good or very poor, depending on how
sample points are allocated to strata. The way to maximize precision through
disproportionate stratification is discussed in a subsequent lesson (see
Statistics Tutorial: Sample Size Within Strata).
If variances differ across strata, disproportionate stratification can provide
better precision than proportionate stratification, when sample points are
correctly allocated to strata.
With disproportionate stratification, the researcher can maximize precision for
a single important survey measure. However, gains in precision may not accrue
to other survey measures.
Recommendation If costs and variances are about equal across strata, choose
proportionate stratification over disproportionate stratification. If the
variances or costs differ across strata, consider disproportionate