Cross-sectional study
- Study design
A snapshot: exposure and disease are measured at the same time in a sample. It gives prevalence, but not which came first.
What it is
In a cross-sectional (prevalence) study a sample of the population is chosen and, at a single point in time, investigators record who has the disease and who has the exposure. It underlies national health surveys and helps plan services. It allows prevalences to be compared between groups, but because everything is measured at once it does not show the time sequence between cause and effect.
How it is built
- The population is defined and a sample is chosen, ideally at random.
- In a single measurement, exposure and the presence of disease are recorded.
- Overall prevalence and prevalence in each exposure group are calculated.
- Groups are compared with the prevalence ratio.
Worked example
- A one-week survey of 1 000 university students: 300 sleep under 6 hours and 700 sleep 6 hours or more.
- Depressive symptoms are present in 60 of the 300 and in 70 of the 700.
- Overall prevalence: (60 + 70) / 1 000 = 130 / 1 000 = 13%.
- Prevalence among short sleepers: 60/300 = 20%; among the rest: 70/700 = 10%.
- Prevalence ratio = 20% / 10% = 2.0.
- Reading: symptoms are twice as common in short sleepers, but we cannot tell whether poor sleep precedes or follows depression.
Strengths and weaknesses
| Strengths | Weaknesses |
|---|---|
| Fast and fairly cheap | Cannot show which came first |
| Measures prevalence and disease burden | Does not measure incidence or risk |
| Studies several exposures and outcomes at once | Over-represents long-lasting diseases |
| Useful for planning services | Open to reverse causation |
Markers
- PR
- Prevalence
When to use it
- National health and nutrition surveys are repeated cross-sectional studies.
- They are used to estimate the prevalence of hypertension, diabetes or anaemia in a community and to plan resources.
Common pitfalls
- Reporting ‘risk’ or ‘incidence’ from a cross-sectional study: what it measures is prevalence.
- Reading a prevalence ratio as causal without thinking about reverse causation.
Test yourself
Think of the answer, then tap the question to see it.
Which measure of frequency does a cross-sectional study give?
Prevalence: the proportion of people who have the disease at the time of measurement.
Why is a cross-sectional study weak for proving causation?
Because it measures exposure and outcome at the same time and does not show that the exposure came before the disease.
Which measure of association is used in a cross-sectional study?
The prevalence ratio (prevalence in the exposed divided by prevalence in the unexposed), or the prevalence odds ratio.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Principles of Epidemiology | Lesson 1 - Section 7 (Centers for Disease Control and Prevention, 2012); Principles of Epidemiology | Lesson 3 - Section 2 (Centers for Disease Control and Prevention, 2012).
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Representative model, for educational purposes only.