Sampling frame Ideally, the sampling frame in a sample survey should list every individual in the population, but in practice, this is often difficult.

Suppose that a sample of households in a community is selected at random from the telephone directory. Explain how this sampling method results in under coverage that could lead to bias.

A random sample of households in a community is chosen from the phone directory.

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 would result in a faulty sample approach, as houses without telephones or whose phone numbers are not included in the telephone directory are excluded from the sampling frame. Under-coverage happens because, due to the selection of a random sample, some groups in the population are left out. This method of sampling is biassed because it does not cover the complete population.