LESSON: Sampling Methods

When it's important for the sample to have members from different segments of the population to be surveyed, you should use a Stratified Random Sample.

In a Stratified Random Sample the population is divided by some characteristic into groups called strata. For example, populations could be divided by gender, age, ethnicity, geographical location or even political preference. Then, a sample is then randomly selected (by simple random sample) from EACH strata. Using a Stratified Random Sample ensures that each segment of the population is represented. 

Let's say an elementary school is going to conduct a survey to decide what foods students want included on the school lunch menu. Elementary schools span a large age group of kids with different likes and dislikes. To ensure that each grade level has an equal opportunity to voice their opinion on the topic the school uses a Stratified Random Sample. The school is divided into strata by grade level. Then, students are randomly selected (by SRS) from each grade level to participate in the lunch menu survey.