A stratified random sample can be obtained by first partitioning a population into disjoint groups (or strata) and then taking random samples from each of the strata. For instance, a stratified random sample can be obtained by first partitioning all the undergraduate students at a university by their class (freshman, sophomore, junior, and senior) and then taking a random sample from each of these strata.

a. Can you think of a situation where a stratified sample may improve on obtaining a representative sample as compared to drawing a simple random sample? Explain.

b. Describe a population and a stratified sample of the population.