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Chapter 35: Biostatistics

Intensive care unit (ICU) delirium is an increasingly recognized problem in ICUs, with multiple risk factors identified. You want to know the prevalence of preexisting cognitive impairment among patients admitted to your ICU. Which of the following types of study designs would be most appropriate to answer this question?

A. Case series

B. Cross-sectional study

C. Cohort study

D. Case-control study

B. Cross-sectional study

Case series describe the characteristics of a disease and are particularly useful to describe rare diseases. They cannot determine the incidence or prevalence of the disease (choice A).

A cross-sectional study determines the number of cases of a disease in a population in a particular point in time and hence can determine the prevalence (choice B).

A cohort study follows a group of participants with a known exposure and a group without the exposure to compare the incidence in both groups (choice C).

A case-control study compares those who already have the disease with matched control participants to retrospectively identify the cause of the disease and hence cannot be used to estimate prevalence of the disease (choice D).

You review different studies regarding ICU delirium to discuss in journal club. All of the following statements regarding studies of different designs are true except

A. Bias is more common in observational studies than in RCTs.

B. Cohort studies are well suited to evaluate rare diseases.

C. Incidence rate can be calculated from cohort studies.

D. Case-control studies fall below cohort studies in the hierarchy of evidence.

B. Cohort studies are well suited to evaluate rare diseases.

Case-control studies are better suited to study rare diseases (choice B). Cohort studies compare the incidence of an event or disease in those with an exposure compared with those without an exposure (choice C). They are useful to evaluate the effect of rare exposures. They, however, are of limited value in evaluating rare diseases because very few participants are likely to develop the disease.

RCTs use methods such as randomization, stratification, and blinding to reduce bias and are higher on the hierarchy of evidence that observational studies (choice A). At the bottom of the evidence pyramid are animal and in vitro studies followed by case reports and series, cross-sectional studies, case-control studies, cohort studies, RCTs, and finally meta-analysis and reviews (choice D).

Match the study ...

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