Where a set numerical formula is used to select the sample, ie every nth person. It is a very simple method to implement. For example, it would involve the researcher being instructed to interview the occupants of every 10th house, or every 20th person who passes by, or to mail questionnaires to every 30th name in a company’s database, or telephone every 50th name in the telephone directory. This method is, however, valid if and only if there is no recurring pattern, for example, every 10th house might be larger. As with simple random sampling it could also miss out a particular group in which the researcher is interested.