QuestionSBA
QuestionChoice
This is a case report, which involves an in-depth study of one to a few individuals with the disease. It is typically used to analyse unusual presentations of disease or unexpected responses to treatment, and is useful with respect to rare or emerging diseases. However, as the sample size is very limited, it would not provide as much data on the various causes of the disease compared to a case-control study
32062
e
A study involving a patient diagnosed with Castleman disease in the university hospital, and an in-depth investigation into his possible predisposing factors to the disease
QuestionChoice
This describes a case-control study, which is a retrospective observational study. It is best suited to study rare diseases for which the population size is very small, and would be able to look at a wide range of exposure factors from the patients' histories
32058
a
A study involving two groups of patients, one with and one without Castleman disease. The groups are then compared to examine their exposure to possible causal factors such as human immunodeficiency virus (HIV) and human herpesvirus 8 (HHV-8)
QuestionChoice
This is a cross-sectional study that provides a snapshot of the disease prevalence and its risk factors in a specific population at one point in time. It would not be suitable in investigating rare diseases as it would be difficult to find a large enough sample size. In addition, it would only be able to identify a potential association between HHV-8 and Castleman disease, but would not be able to define a direct cause and effect relationship
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c
A study looking at the prevalence of Castleman disease in all males aged 20-50 years old in London and finding out whether they have been exposed to human herpesvirus 8 (HHV-8)
QuestionChoice
This describes a prospective cohort study, a longitudinal study that observes participants over a period of time to see whether they develop the outcome in question. This would not be suitable in this case as the disease is very rare, and the time taken to develop the condition may be very long
32059
b
A study involving two groups of patients, one infected with human immunodeficiency virus (HIV) and the other without. The groups are then followed up over a period of 5 years to determine whether or not they develop Castleman disease
QuestionChoice
This is a retrospective cohort study, which is useful for tracking the development of a disease with a long latency period. It is different from a prospective cohort study in that study groups are not followed up in the future. It would not be suitable in this case as the disease is very rare
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d
A study involving previously collected data to investigate whether there was an association between patients infected with human immunodeficiency virus (HIV) and their development of Castleman disease over a five year period
QuestionComment
I thought that causality couldn't be inferred from case-control studies? Also, the requirement to know that Castleman disease is rare seems like an unnecessary knowledge requirement in a Q that is testing study design
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**Observational Study**

A type of study in which participants in two or more groups are observed without the addition of an intervention. This is in contrast to randomised controlled trials where researchers look at the effects of a specific intervention in the study groups.

**Cohort studies**

Cohort studies are one type of observational study. This study design involves examining groups of people over a period of time, where one group is exposed to a certain risk factor and the other group is not in order to establish links between exposure and a health related outcome such as the development of disease.

These designs are usually prospective in nature however, can be conducted retrospectively by looking back at data already collected, such as hospital records. These studies are useful to measure incidence and risk however, they are prone to confounding factors and are often costly.

**Case-control studies**

Case-control studies examine an association between an outcome and exposure to a risk factor. The studies recruit participants based on the presence of an outcome from the outset of the study. Once cases are recruited, suitable controls are identified and selected for the study. Controls are participants without the outcome of interest. Data of exposure to a risk factor is recorded retrospectively for each of the participants and then compared between cases and controls.

These studies are more suited to investigate rare conditions as individuals with the outcome of interest are identified at the outset. Other advantages of case-control studies are that they are much quicker to carry out relative to cohort studies and are relatively inexpensive.

**Bradford-Hill Criteria**

The Bradford-Hill criteria include 9 established viewpoints to help determine if observed epidemiologic associations are causal.
Hill advised that his following viewpoints of an association be considered in attempting to distinguish casual from non-casual associations: strength, consistency, specificity, temporality, biologic gradient, plausibility, coherence, experimental evidence and analogy.

**Strength** - strong associations are more likely to be casual than weak associations (which may be explained by other factors or be affected by undetected biases.)

**Consistency** - refers to the repeated observation of an association in different populations under different circumstances. Lack of consistency does not rule out a casual association, because some effects are produced by causes only under unusual circumstances.

**Specificity** - The criterion of specificity requires that a cause leads to a single effect, not multiple effects. Hill's discussion of this criterion is with reservation, and many authors regard this criterion as useless and misleading on the basis that many outcomes can happen from one variable (e.g. smoking may cause lung cancer, COPD, other forms of cancer, interstitial lung disease..)

**Temporality** - Temporality refers to the necessity that the cause precede the effect in time.

**Biologic gradient** - Biologic gradient refers to the presence of a mono-tone (unidirectional) dose responsive curve. However, like specificity, the acknowledgement that disease frequency may be caused by different exposures shows that the existence of a monotonic association is neither necessary nor sufficient for a causal relation.

**Plausibility** - Plausibility refers to the biologic plausibility of the hypothesis, an important concern but one that is far from objective or absolute.

**Coherence** - implies that a cause and effect interpretation for an association does not conflict with what is known of the natural history and biology of the disease. On the other hand, presence of conflicting information may indeed undermine a hypothesis, but one must always remember that the conflicting information may be mistaken or misinterpreted.

**Experimental evidence** - it is not clear what was meant by this criterion, however, it may have referred to errors when collecting data.

**Analogy** - Analogy provides scientists with a source of more elaborate hypotheses about the associations under study; absence of such analogies only reflects lack of imagination or experience, not falsity of the hypothesis.

[Source: https://www.rtihs.org/sites/default/files/26902%20Rothman%201998%20The%20encyclopedia%20of%20biostatistics.pdf]
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Medical Statistics
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A case-control study, a type of retrospective observational study, is ideal for studying rare diseases or outcomes with a small population size. It allows researchers to examine a wide range of exposure factors from patients' histories to identify potential associations between exposures and the disease.
A university professor would like to conduct a study to investigate the causes of Castleman disease, a disease involving enlarged lymph nodes in the body.

Which is the most appropriate study design he should select?
a