Systematic sampling is a random sampling technique which is frequently chosen by researchers for its simplicity and its periodic quality.
In systematic random sampling, the researcher first randomly picks the first item or subject from the population. Then, the researcher will select each n'th subject from the list.
The procedure involved in systematic random sampling is very easy and can be done manually. The results are representative of the population unless certain characteristics of the population are repeated for every n'th individual, which is highly unlikely.
The process of obtaining the systematic sample is much like an arithmetic progression.
The integer is typically selected so that the researcher obtains the correct sample size
For example, the researcher has a population total of 100 individuals and need 12 subjects. He first picks his starting number, 5.
Then the researcher picks his interval, 8. The members of his sample will be individuals 5, 13, 21, 29, 37, 45, 53, 61, 69, 77, 85, 93.
Other researchers use a modified systematic random sampling technique wherein they first identify the needed sample size. Then, they divide the total number of the population with the sample size to obtain the sampling fraction. The sampling fraction is then used as the constant difference between subjects.