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When you’re conducting a study, a very important decision you will be making is how you will sample (i.e., obtain) participants or subjects. The sampling method (random sampling, systematic sampling, etc.) you use can directly affect how reliable and generalizable your results are. It can also affect your study budget, choice of data analysis method, etc.
Today we’ll dive into one popular sampling method: systematic sampling. If you want a broad overview of all the sampling methods available, see this article. We also have some tips on choosing the right sampling method.
What is systematic sampling?
Systematic sampling involves selecting every “n”th item from a list of the study population. So if you’re conducting a study on burnout in nurses, and select every 4th nurse from the personnel register at X hospital, you’re using systematic sampling.
Advantages of systematic sampling: when to use it
Systematic sampling is easy to do and is efficient. It’s also cost-effective. So, opt for it if
- You’ve a tight budget
- You’re short on time
But note this key disadvantage: you could end up with bias if there is an underlying pattern in the population list. Like in the example of hospital nurses above, if the list was deliberately ordered “male, female, male, female,” you’d end up missing the male nurses in your sample.
Steps in systematic sampling
As mentioned earlier, systematic sampling is quite easy and straightforward.
- Confirm the desired sample size: Make sure you’ve a sufficiently big sample, by running a power analysis and estimating effect size. [This handy infographic can help!]
- Decide the sampling interval: One simple way of doing this is to divide the total study population by your desired sample size.
- Select a random starting point: Be sure your starting point is randomly chosen, to lower the risk of bias.
- Start sampling!
Types of systematic sampling: Circular vs linear
As you move down the population list, you’ll ultimately reach the end. What do you do? If you stop sampling, you’re using linear systematic sampling. If you go back to the beginning and move back down again, excluding those you’ve already selected, you’re using circular systematic sampling. Circular systematic sampling is easier when
- Your study population is limited
- You do not want to compromise on a smaller sample size
