Which study design you are actually running
Observational or interventional; and if observational, which direction time runs in.
Observational — you watch and record; you change nothing. A *cross-sectional* study measures everything at one point in time. A *cohort* study follows people forward and sees who develops the outcome. A *case-control* study starts from people who already have the outcome and looks backward for exposures.
Interventional — you do something and see what happens. A *randomised controlled trial* allocates participants to groups by chance. A *before-and-after* study measures the same people twice, with your intervention in between.
Why it matters
The design determines what you may claim. A cross-sectional study can show that two things occur together; it cannot show that one caused the other, because you did not observe which came first. It also determines the statistics: two independent groups are compared differently from the same group measured twice, and using the wrong one inflates or destroys your significance.
An example
Same topic, four designs:
• Cross-sectional — measure vitamin D in 200 nursing students once, and report the prevalence of deficiency.
• Case-control — take 60 students with deficiency and 60 without, and compare their sun exposure.
• Cohort — follow 200 students for a year and see who becomes deficient.
• Trial — randomise 100 deficient students to supplementation or no supplementation, and compare levels at six months.
Common mistakes
- Calling a before-and-after study a trial. Without a control group you cannot separate your intervention from time, practice or regression to the mean.
- Claiming causation from a cross-sectional study.
- Treating paired measurements as independent groups, which throws away the pairing and usually the significance with it.
Read next
- Paired or independent — the question that changes the test — Are your two sets of numbers from the same people, or different people?
- A research question you can actually answer — The difference between a topic and a question, and why the question decides everything that follows.
Chavery Research Companion applies this to your own study: it asks the questions in plain language, checks the assumptions against your data, recommends the test, and writes the sentence that reports it. Start free — planning and the master chart cost nothing.