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Random digit dialing equipment, which chooses the final four digits of a telephone number at random after being given the exchange, is more commonly used in telephone surveys (the first three digits).

When random sampling is not used, bias or systematic flaws in the way the sample represents the population can occur. Voluntary response samples, in which respondents choose their own respondents, and convenience samples, in which people who live nearby are included in the sample, are particularly biased.

This sample approach excludes households in the community that do not have telephones. Households without a telephone would be poor, and those who cannot afford a phone would be poorer than those who just use cell phones and do not have a phone. Under-coverage happens because, due to the selection of a random sample, some groups in the population are left out. Bias results from this sampling inaccuracy.