Mary Hall is a editor for Investopedia's Advisor Insights, in addition to being the editor of several books and doctoral papers. Mary received her bachelor's in English from Kent State University with ...

Melissa Horton is a financial literacy professional. She has 10+ years of experience in the financial services and planning industry. Robert Kelly is managing director of XTS Energy LLC, and has more ...

Stratified random sampling - random samples are taken from within certain categories. Stratified systematic sampling - regular samples are taken from within certain categories.