A qualitative study of the illness of a single patient or a group of other patients diagnosed with the same or a similar disease is known as Case Series. It may also refer to techniques that are used to compare the periods of exposure to an agent having the potential to cause the illness to periods during which they remain unexposed.
The first type of study is mainly descriptive, and the inferences obtained from them cannot give us an idea of the general population of people having or succumbing to these diseases.
Two types of Studies
In these studies, a new hypothesis can be formed if astute clinicians notice any abnormality in the patient’s history or the growth of the disease. The data obtained can help to analyze the situation, and further investigations may reveal the causes of the disease. These are generally case-control studies or prospective studies.
The former involves matching the comparable controls in the absence of the disease to the cases already present in the series. Prospective studies tend to supervise the case series over a particular period to evaluate the natural history of the disease. Prospective studies were formally called as self-controlled studies.
They divide patient follow up time into unexposed and exposed periods and compare the outcome between the two. Adverse reactions to vaccination were studied extensively by the use of this technique, and this has given us more leverage over the diseases than cohort studies.
The disease statuses of individuals are used to select subjects for case-control studies. Disease positive groups (case group) are compared with individuals who are disease negative (control group). Both groups should be ideally selected from the same population. The study looks back through time to understand the potential exposures that the groups might have faced.
This exposure is what gives rise to the cases. Though these studies are faster and cost-effective, they are susceptible to certain types of bias, like recall bias and selection bias. The challenge in these studies is to identify appropriate control groups, which give us an idea of the overall distribution of exposure among the population.
This is done by drawing a sample from the original population, which presents the leading risk factor. The consequence of this is that the control group contains people suffering from the disease even as the condition is at high risk of infecting the given population.
These studies select individual subjects based on either the level of exposure they face or their exposure status. The study subjects will be at the risk of getting the disease in case of an outbreak. This implies that they should be free of the disease when the studies start.
For example, a cohort study of smokers and non-smokers is done to estimate the chance of any one individual of the population getting lung cancer over some time. Prospective studies are known to be more beneficial than cohort studies, but the possibility of losing subjects during follow up periods makes it a higher risk.
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