Situation: A public health nurse is assigned to organize and assess a coastal barangay in a municipality with a midyear population of 40,000. A cross-sectional survey in the barangay found that households with uncovered water containers reported more dengue cases in the past year. Which conclusion is BEST supported by this finding?

A cross-sectional survey measures exposure and outcome at the same time. It can show that the two are associated, but not that the exposure came before the illness (temporality), so it cannot establish causation. Proving that covering containers prevents dengue needs a stronger design, such as a trial.

A cross-sectional survey captures information about both the exposure and the outcome at a single point in time. In this barangay assessment, the nurse collected data on whether households had uncovered water containers and whether those households reported dengue cases during the past year, all within the same survey period. Because both pieces of information were measured simultaneously, the finding that households with uncovered containers reported more dengue cases demonstrates a statistical association, but it does not establish the order in which the exposure and the outcome occurred.

The central limitation is temporality, which is one of the criteria needed to move from association toward causation. A cross-sectional design cannot determine whether the uncovered water containers were present before the dengue infections, or whether households with dengue cases later developed conditions that led to uncovered containers. Without knowing which came first, the survey cannot support a causal claim.

This distinction matters because dengue prevention programs often target water containers as breeding sites for Aedes aegypti mosquitoes. The baseline data from a cluster-randomised controlled trial in Cucuta, Colombia, collected cross-sectional information on water containers and arbovirus transmission risk precisely because such baseline data can identify associations and guide intervention design, but the trial itself was needed to test whether modifying those containers reduces disease ^[1]. The cross-sectional component alone could not prove that containers caused transmission.

Observational studies with stronger designs, such as case-control studies, can also identify risk factors but still face limitations in establishing temporality. A case-control study in Mogadishu, Somalia, identified risk factors associated with dengue fever by comparing cases and controls, yet the design remains retrospective and cannot fully resolve the sequence of exposure and outcome ^[2]. Similarly, a case-control study in Rawalpindi, Pakistan, examined risk factors during a dengue outbreak and recommended preventive measures, but the authors relied on the epidemiological pattern and prior knowledge rather than the study design alone to infer causality ^[3]. A case-control study in Fortaleza, Brazil, found that receptacles in the garden were significant risk factors for dengue during an outbreak, but again, the design showed association rather than direct proof that the receptacles preceded every infection ^[4].

Watch out! Option 1 incorrectly states that the survey confirms the containers came first. A cross-sectional survey has no mechanism to verify temporal sequence. Option 4 makes a direct causal claim that the survey cannot support. Option 3 is too absolute: a cross-sectional survey can show an association, even though it cannot establish causation. Key point! Cross-sectional studies measure prevalence and can reveal associations, but they cannot establish temporality or causation. Proving that covering water containers prevents dengue requires a stronger design, such as a randomised controlled trial or a well-designed cohort study that follows households over time.