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)
- 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
- Susceptible to recall bias or information bias
- Less control over variables
- Low cost
- No follow-up required
- Provides a rationale for conducting cohort studies
- Unable to determine cause and effect
- Rare conditions cannot be studied
- Good for examining rare outcomes
- Quick to conduct
- 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
- Legangie P. Application and interpretation of simple odds ratios in physical therapy-related research. J Orthop Sports Phys There 2001;31:496-503.
- Mann C. Observational research methods. Research design II: cohort, cross sectional, and case-control studies. Emerg Med J 2003;20:54-60.
- Song J, Chung K. Observational studies: Cohort and case-control studies. Plast Reconstr Surg 2010;126:2234-2242.