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A statistical unit is the unit of observation (e.g. entity, person, object, product) for which data are collected. For each statistical unit, qualitative or quantitative variables are observed or measured.

The whole set of statistical units is the population. It may also be virtually infinite (e.g. all products of a production process).

A sample is a subset of statistical units (sampling units) selected from the population in a suitable way. The sample size of a study is the total number of sampling units (see Figure 1.1).

Figure 1.1 Population, samples, sampling units.

When we use a sample to draw conclusions about a population, sample selection must be performed at random. Random sampling is carried out in such a way as to ensure that no element in the population is given preference over any other. Random sampling is used to avoid nonrepresentative samples of the population.

In Example 1.1 the statistical unit is the single respondent.