July 3, 2017

Observational Studies

The purpose of cohort studies is to describe the incidence and natural history of a disease or condition. Incidence is the rate of new cases of a condition that develop over a period of time.

Advantages of cohort studies
  • Gather data regarding sequence of events
  • Can assess causality
  • Examine multiple outcomes for a given exposure
  • Good for investigating rare exposures
  • Can calculate rates of disease in exposed and unexposed individuals over time (e.g., incidence, relative risk)

Disadvantages of cohort studies

  • Large number of subjects are required to study rare exposures
  • Susceptible to selection bias

There are two types of cohort studies; prospective and retrospective. Prospective cohort studies monitor a group of individuals, who may or may not have been exposed to a disease, over a period of time. A number of relevant variables are monitored over the same period of time in order to determine if there is a relationship between exposure and disease that has already been collected through previous studies or through medical charts.

Disadvantages of prospective cohort studies
  • May be expensive to conduct
  • May require long durations for follow-up
  • Maintaining follow-up may be difficult
  • Susceptible to loss to follow-up or withdrawals

Disadvantages of retrospective cohort studies
  • Susceptible to recall bias or information bias
  • Less control over variables

Cross-sectional studies are used to determine the prevalence of a condition. Prevalence is the number of cases in the population at a given time. Therefore, a cross-sectional study can be considered a one-time snapshot of the sample or population.

  • Quick
  • Low cost
  • No follow-up required
  • Provides a rationale for conducting cohort studies

  • Unable to determine cause and effect
  • Rare conditions cannot be studied

Case-control studies are designed to investigate rare conditions. Subjects are identified by outcome status at the beginning of the study. They are then divided into two groups, cases and controls. Data with respect to risk factors are collected retrospectively.

  • Good for examining rare outcomes
  • Quick to conduct
  • Inexpensive
  • Existing records can be used
  • Few subjects
  • Multiple exposures or risk factors can be examined

  • Recall bias
  • Information bias
  • Difficult to validate information
  • Cannot account for confounding variables
  • Selection of appropriate comparison group may be difficult
  • Rates of disease in exposed and unexposed individuals cannot be determined

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  3. Song J, Chung K. Observational studies: Cohort and case-control studies. Plast Reconstr Surg 2010;126:2234-2242.