Revision notes · Lecture 5 · Free
Descriptive study designs
In one line: descriptive studies answer who, where and when, never why. They describe one group, with no comparison group.
What this lecture's questions test
Out of the 30 questions on this lecture:
| Idea | Questions |
|---|---|
| Cross-sectional studies | 13 |
| Case reports and case series | 8 |
| Ecological studies | 5 |
| Descriptive or analytical? | 4 |
1. Descriptive or analytical?
- Descriptive: person (who), place (where) and time (when). It cannot answer why, and it cannot identify risk factors.
- Comparing two groups makes a study analytical.
- Measuring incidence needs a disease-free group followed over time. That is a cohort study, not a descriptive one.
2. The four descriptive designs
| Design | Studies | Best for |
|---|---|---|
| Case report | One patient | A rare or new presentation, an unexpected drug effect |
| Case series | A group of similar patients | Uncommon clinical features. No comparison group |
| Cross-sectional | Individuals, at one point in time | Prevalence, burden, generating hypotheses |
| Ecological | Groups: districts, states, countries | Relating disease rates to exposure rates, generating hypotheses |
Remember it: one patient → report. Several → series. A snapshot of a population → cross-sectional. Whole populations compared → ecological.
- A case report cannot give incidence: there is no population to divide by.
3. Cross-sectional studies
- Exposure and outcome are measured at the same time: a snapshot.
- The unit of observation is the individual. It counts all existing cases, old and new, not only new ones.
- Uses: burden (prevalence), trends, generating hypotheses. It can measure several outcomes at once. A census is a cross-sectional survey.
- Main limitation: it cannot show which came first (temporality), so it cannot establish cause.
- The analytical cross-sectional study compares groups within the snapshot, and so can identify factors associated with an outcome.
Remember it: association yes, sequence no.
4. Ecological studies
- The data are group-level: exposure and outcome for each district or country, never for each person.
- Strength: cheap and quick, using data that already exist. It relates rates and generates hypotheses.
- Weakness: it cannot link one person's exposure to that person's outcome. Assuming it can is the ecological fallacy.
- Example: tobacco sales and oral cancer rates, state by state.
Watch out: one true/false question asks whether a descriptive cross-sectional study has a comparison group. The key and standard teaching disagree; the app explains it on that question.
Last look before the exam
- Descriptive: who, where, when, not why.
- One case → report. A group → series. Neither has a comparison group.
- Burden or prevalence → cross-sectional.
- Cross-sectional cannot show temporality.
- Group-level data → ecological.