Types of Sampling

Some Vocabulary  Homogeneous groups: All members of the group have a characteristic that is the same.  Heterogeneous groups: all members of the group have characteristics that differ but make up the characteristics of the entire population

Stratified sampling  Split the population into homogeneous groups before selecting a sample.  Then use simple random sampling within each strata to make a larger sample.

Stratified sampling Example  Split the school into grades (each grade is a strata)  Do a simple random sample to choose 10 people from each grade.  Combine the 10 people from each grade to make a random sample of 40 people from the school.

Cluster Samples  Split the population into heterogeneous groups (clusters).  Randomly select a cluster (or a few clusters) to make up your sample of the population.  Take a Census of that cluster.

Cluster Sampling Example  When studying ages of doctors in Delaware…  Each hospital is a cluster.  Select a hospital as your sample and survey ALL of the doctors in that hospital.

Stratified vs. Cluster  Think of a Boston cream pie which consists of a layer of yellow cake, a layer of crème, and a layer of another cake, and then chocolate frosting…

Layered Cake  Cluster: Taking a vertical slice of the cake.  Learn about the whole pie and all of its layers combined by taking that vertical slice  Strata: take a random piece of yellow cake, random piece of chocolate, random piece of crème.  Get an idea of the entire cake from parts of each horizontal layer

Systematic Sampling  Put all people in the population in a random order and then select every n-th member.  Careful: Order of the list must not be grouped in any way related to the study.

Systematic example  Put the school in alphabetical order and select every 20 th name.