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Chapter 1: Introduction and Graphical Displays
affiliation, classifications of university students, gender, ethnicity, etc. In
cases like these, the population is divided into subgroups or strata and the
sample is obtained from this structure.
Definition: Stratified Sample
A stratified sample is obtained by dividing the population into homogenous
subgroups or strata based on some characteristic (e.g. gender, race, etc.) and
then a random sample is selected from each stratum. These random samples
are analyzed individually and then combined to obtain an overall sample
from the population to be analyzed.
Figure 1- 6
shows an illustration for a stratified sample with three strata.
Figure 1- 6:
Illustration for stratified sampling with three strata
There are some advantages and disadvantages of selecting a stratified
sample.
Advantages of a Stratified Sample
It concentrates on important subgroups and ignores unimportant
ones.