Before moving on to the next example, we will introduce a number of terms that will turn the solution of the next example into a child’s play. In presenting the terms, we will use the previous example, in which we calculated the average grade in the class:

1. Target population – all of the pupils (for whom we calculate the average) are called the target population.

2. Individual – everyone in the target population is called an individual.

3. Samples – in general, the pupils’ marks are called samples.

4. Sample data – the specific grades of every pupil. In our example, there are only three sample data: 90 pts., 80 pts., and 70 pts.

Had we calculated the average height of the class’s pupils, then the samples would have been the pupils’ heights, and the sample data would have indicated the height measurements of all the pupils.

In all of the following examples, we will first identify the target population and the sample data. We will then arrange the target population in groups according to the sample data. All the individuals with the same sample data belong to the same group.