A stratified sample

A stratified sample is obtained by independently selecting a separate simple random sample from each population sub-group. A population can be divided into different groups based on some characteristic or variable that is important to your research question like income or level of education. To employ a stratified random sample select from a proportionate number of participants from each stratum. For example group A has 100 persons while group B has 50, and C has 30 you may decide you will take 10% of each. So you end up with 10 from group A, 5 from group B and 3 from group C.