## Core Concept
Cohort studies are a type of observational study design used in epidemiology and medical research. They involve following a group of individuals (the cohort) over time who share a common characteristic, such as exposure to a potential risk factor or a specific disease. The goal is to observe how the exposure affects outcomes, such as the development of a disease.

## Why the Correct Answer is Right
The correct statement about cohort studies, which is not provided, generally highlights a key feature such as prospective or retrospective follow-up, the assessment of exposure and outcome, or the calculation of incidence rates. Typically, cohort studies are useful for studying rare exposures, understanding the natural history of diseases, and generating hypotheses about disease causation.

## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific details of option A, a general approach is to consider what would make a statement about cohort studies incorrect. For instance, if option A states that cohort studies are used to study common diseases, this might not necessarily be false but could be misleading because cohort studies are particularly valuable for studying rare exposures or outcomes.
- **Option B:** Similarly, if option B suggests that cohort studies can establish causation, while they can provide strong evidence for associations and temporality (a crucial aspect of causation), establishing causation requires careful consideration of multiple factors, including confounding variables.
- **Option C:** If option C mentions something about the cost or duration of cohort studies, it could be incorrect if it underestimates or overestimates these aspects. Cohort studies, especially prospective ones, can be long-term and expensive.
- **Option D:** If option D discusses the calculation of relative risk or the ability to study multiple outcomes from a single cohort, incorrect statements might involve misunderstandings about the types of risk assessments cohort studies can provide.

## Clinical Pearl / High-Yield Fact
A key point to remember about cohort studies is that they are particularly useful for **prospective** studies aiming to assess the **incidence** of diseases and can help in understanding **temporality** and potential **causality** between exposure and outcome. A classic example or a point of confusion often revolves around the difference between **cohort studies** and **case-control studies**, with the former being more resource-intensive but offering clearer temporal relationships.

## Correct Answer: D.