Systematic sampling

What is systematic sampling?

Systematic sampling is a type of probability sampling that it is based on listing an entire population, randomly choosing the first individual for the sample and then, from an interval defined by the researcher, select the rest of the individuals that will make up the sample.

For example, suppose we need extract a sample from 10 people from a total population of 100 and the first individual selected for the sample is number 3. From this, through an interval of 4 decided by the researcher, the next individuals will be selected until the sample is completed, so that they will be numbers 7, 11, 15, etc.

Systematic sampling example with interval of 4.

Advantages and disadvantages of systematic sampling

Systematic sampling has both advantages and disadvantages when implementing it.

Advantage

Its main benefits are the following:

• Allows to obtain good representativeness properties.
• It's a very fast and simple system, since it avoids the need to generate random numbers of individuals in the sample.
• Guarantees a perfectly fair selection of the population, which is very useful in distinguishing groups.
• Avoid the need to use different layers, although there is a variety of individuals in each fragment.

Disadvantages

There is only one disadvantage or inconvenience when using this technique: the order in which the selected population is placed may have a hidden periodicity type, which may coincide with the selected interval, causing a sample biased.

Examples of systematic sampling

Suppose we have a population of 1000 individuals and we need to obtain a sample of 100 of them. For it, we will first divide the total population into 100 random fragments of 10 individuals. Then we will select a random number between 1 and 10.

If the number obtained at random is 6, our sample will be defined from individual number 6 of the population. That is to say that from it intervals of 10 will be completed, that is: 6, 16, 26, 36, 46 until 996.

In this way the sample of 100 individuals will be made up through systematic sampling.