Disclaimer: This article is primarily intended for struggling learners. As such, there is some deliberate oversimplification. However, the guidance provided is of a general nature, and may be of use to others as well.
Note: You may freely use and share the algorithm provided in this article for non-commercial purposes provided you mention the source (watermark notwithstanding).
In a previous article, I have provided guidance on how to identify the study design in a theory question. This article is primarily aimed at struggling students, who may be unable to employ the strategies mentioned in that article.
Broadly, study designs may be either observational or experimental. At the MBBS level, students have to choose between Case Control and Cohort studies (observational); and Randomized Controlled Trials (RCTs) (experimental).
In observational studies, exposure to suspected risk factor(s) is not determined by the investigator. In contrast, it is the investigator who determines exposure in experimental studies (or at least, the study subjects do not have a choice once they give their written informed consent to participate in the study).
Observational studies are suitable for generating hypotheses, and narrowing the scope of future investigation(s). Experimental studies are used to test hypotheses, and are subjected to very strict regulations.
Questions pertaining to study design are often essay questions, with the main body of the question providing some details, and sub-questions that follow from the main body.
Main body of question
The first step in identification of the study design is reading the main body of the question. Here, there are often vital clues regarding the sub-questions that follow.
All questions related to study designs will mention some disease. However, the manner in which disease is mentioned yields clues about the study design. There are mainly three variants of such questions:
- The main body mentions a disease outbreak
- The main body mentions a drug/ intervention to manage a disease
- The main body mentions a risk factor/ exposure and occurrence of a disease
Let us consider each variant separately:
The main body mentions a disease outbreak
The instinctive response to a question mentioning disease outbreak is to assume one needs to describe ‘Investigation of an epidemic’. However, here the question is about the appropriate study design, NOT investigation of an epidemic.
One might wonder how a study design would be relevant in an outbreak. However, the question is not flawed. Any investigation of an outbreak would require an investigation of factors that lead to the outbreak. This would require a comparison between the characteristics of those affected by the outbreak, with those not affected. The investigation must be completed within a short time period, too. Under such circumstances, the Case control study is best suited to yield answers regarding the risk factors for disease among those affected. Therefore, where the question mentions disease outbreak, the appropriate study design is Case control study.
The main body mentions a drug/ intervention to manage a disease
Sometimes, the main body of the question will mention a novel therapy/ drug for a management of a particular disease, indicating that the new modality must be tested. This requires testing the new therapy under controlled conditions in order to definitively establish the effectiveness (or lack of it) of the new therapy. Such conditions can only be ensured in an experimental study, and a Randomized Controlled Trial would be the most appropriate study design in such situations.
The main body mentions a risk factor/ exposure and occurrence of a disease
Some questions mention exposure to a risk factor and occurrence of a related disease. In such situations, one must try to establish whether the disease has already occurred or not. This is often evident from the choice of words used in the question. Where the disease has already occurred, one may describe a Case control study. Where disease has not yet occurred, a cohort study would be appropriate. In questions where it is not clear if disease has occurred, or will occur in future, one may describe either Case control or Cohort study design provided appropriate justification is supplied. Thus, if one describes Case control study in an ambiguous question, one should state that the study will involve a comparison between cases and controls. Similarly, if stating Cohort study, one must mention that disease free study subjects will be followed up over time. Randomized Controlled Trials (RCTs) would generally be inappropriate in these circumstances due to ethical considerations, and can be safely omitted.
Please note that as long as there is reasonable (logical) justification for a study design, candidates will not be penalised.
Often, the sub-questions provide additional clues to the appropriate study design, and must be carefully screened for key words.
Generally, sub-questions may include one or other of three words/ phrases:
- Limited time for study
- Bias/ Matching
- Randomization/ Blinding
Limited time for study
Various approaches may be used to indicate a limited time-frame for the study. Sometimes the question states that the study must be completed within a particular clinical posting. Alternatively, the question may state a short time period- 2 months, for instance. Candidates must determine if the choice of words indicate a short time period for conduct of the study. As mentioned above, where time is limited, Case control studies are appropriate.
Where the sub-question mentions the term ‘Bias’ or ‘Matching’, one can be certain the appropriate study design is Case control study, as these terms occur in connection with that study only.
Where the sub-question mentions the term ‘Randomization’ or ‘Blinding’, one can be certain the appropriate study design is Randomized Controlled Trial (RCT), as these are procedures exclusively performed in connection with Clinical Trials.
Link to previous article on identification of study design: