Core question
The nurse is using data that were already collected in the past—household water-container status from a survey
2 years ago—and then checking subsequent clinic records to see which children later developed dengue. The key is the direction of observation: the study starts with
exposure status and follows forward in time to the
outcome.
Why this is a cohort study
In a
cohort study, participants are grouped according to whether they have a particular exposure—here, living in a home with an uncovered water container—and are then observed over time to determine who develops the outcome of interest, in this case dengue. Because the exposure was recorded in the past and the outcome data already exist in clinic records, this is specifically a
retrospective cohort study, also called a historical cohort study. The investigator is not intervening or assigning exposure; the exposure status was already present in the household survey data.
Starting from exposure and following forward to outcome defines a cohort design, even when both the exposure and outcome data come from existing records. The temporal sequence is preserved: the water-container status was documented first, and dengue occurrence was assessed afterward.
Why the other options are incorrect
A
case-control study works in the opposite direction. It begins by identifying people who already have the outcome—children with dengue—and then looks backward to compare their past exposure to uncovered water containers with that of children who did not develop dengue. Here, the nurse starts with exposure status, not disease status, so this is not case-control.
A
cross-sectional study measures exposure and outcome at the same point in time. It can show an association but cannot establish that the exposure preceded the outcome. In this scenario, the water-container survey was done
2 years ago, and dengue was assessed afterward, so the design is longitudinal rather than cross-sectional.
A
randomized controlled trial requires the investigator to randomly assign participants to an intervention or control condition. The nurse is only observing existing data and is not assigning any household to have covered or uncovered water containers.
Connecting to the evidence
The cited cohort study from Vietnam illustrates the same logic in a real epidemiological setting. Investigators used geo-referenced household data to classify exposure—lack of tap water and the resulting need to store water in containers—and then followed the population over time using dengue hospital admissions as the outcome.
That study demonstrates how a cohort design can link household water-storage conditions to subsequent dengue risk without any experimental intervention. The exposure was an existing household characteristic, and the outcome was ascertained from health records, matching the retrospective cohort structure described in this question.
Watch out! The phrase “household survey done 2 years ago” can make this look like a case-control study because the data are old. The distinguishing feature is not the age of the data but the direction of analysis.
Key point! If you start with exposure and move forward to outcome, it is a cohort study—retrospective if the data already exist, prospective if you follow participants into the future.
| Design | Starting point | Direction of observation | Typical use |
|---|
| Cohort | Exposure status | Forward to outcome | Estimate incidence or risk over time |
| Case-control | Outcome status | Backward to exposure | Study rare diseases efficiently |
| Cross-sectional | Exposure and outcome at same time | Snapshot only | Prevalence and association |
| Randomized controlled trial | Investigator-assigned intervention | Forward to outcome | Test treatment or prevention effect |
Clinical and public health nursing relevance
For a public health nurse, recognizing a retrospective cohort design matters because it uses existing surveillance or survey data to answer questions about whether a household-level exposure—such as uncovered water storage—predicts disease. In dengue-endemic areas,
Aedes aegypti mosquitoes breed in stored water, and identifying homes with uncovered containers allows the nurse to target health education and vector-control measures. The cohort design supports the inference that reducing uncovered water storage may lower dengue risk because the exposure was documented before the outcome occurred.