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  • The purely random sample is the only kind that can be examined with confidence by means of statistical theory, but there is one things wrong with it. It is so difficult and expensive to obtain for many uses that sheer cost eliminates it. A more economical substitute, which is almost universally used in such fields as opinion polling and market research, is called stratified random sampling.

    Darrell Huff (2010). “How to Lie with Statistics”, p.23, W. W. Norton & Company