Case Report vs Cross-Sectional Study: A Simple Explanation

A case report is the description of the clinical story of a single patient. A cross-sectional study involves a group of participants on which data is collected at a single point in time to investigate the relationship between a certain exposure and an outcome.

Here’s a table that summarizes the relationship between a case report and a cross-sectional study:

Case ReportCross-Sectional Study
Participants involvedA case report describes the medical case of 1 particular patientA cross-sectional study is a snapshot in time of a sample of participants chosen from the population
GoalTo report an interesting or unusual case of a patientTo describe the association between an exposure and an outcome
Study typeObservational
(because the researcher only observes and describes the patient’s case and does not manipulate or control the events)
(because the researcher does not influence who gets the exposure and who doesn’t)
Follow-up over timeYes, sometimes the case report involves following the patient over a period of timeNo, in a cross-sectional study the exposure and the outcome are measured simultaneously and so there is no need to follow participants over time
ExampleIn 1991, Fred Kern, Jr. reported the case of an 88-year-old man who has been eating 20-30 eggs each day for almost 15 years. The man had a normal cholesterol level as his body adapted to his unusual diet. [Source]In 1999, Von Kries et al. measured, at the time of school entry, the weight of children and whether or not they had been breastfed and for how long. The study concluded that prolonged breastfeeding (the exposure) may decrease the prevalence of childhood obesity (the outcome). [Source]
AdvantagesSimple and inexpensive: as it involves following 1 patient only– Fast to conduct: because participants are not followed over time
– Useful for assessing the disease burden in the population
LimitationsIs considered a weak design because:
– It represents a single story that does not always generalize to other cases.
– Conclusions based on case reports may be biased (because the observed patient is not chosen at random from the population) or confounded by some unmeasured factors.
Is a weak design for assessing a causal relationship between exposure and outcome because of:
Reverse causality: As the exposure and the outcome are measured at the same time, it is hard to determine which came first.
Survival bias: The study may be excluding those who died from the exposure (especially those who died after being exposed but before the study was carried out), therefore the results may be biased towards concluding that the exposure is less harmful than it really is.
Level of evidenceHas the lowest level of evidence of all study designsIs one step better than the case report

Further reading