M09.01.13 Cohort study design

In epidemiology, a cohort study examines the association between exposure to a risk factor and the development of a disease. This guide outlines the core metrics used in cohort studies—relative Risk (RR) and Tributable Risk (AR)—to help quantify and interpret the risk of disease in exposed and unexposed groups.

Key Terms and Definitions

1. Relative Risk (RR): Measures the strength of the association by comparing the incidence rate of disease in the exposed group to the incidence rate in the unexposed group.
   • Question: How much more likely is one group to develop the disease compared to another?
   • Formula: RR=Incidence in Exposed GroupIncidence in Unexposed Group
     • RR =
2. Attributable Risk (AR): Measures the actual difference in incidence between exposed and unexposed groups, identifying how many additional cases occur due to exposure.
   • Question: How many more cases are observed in the exposed group compared to the unexposed?
   • Formula:
     • AR =
3. Number Needed to Harm (NNH): The number of individuals who need to be exposed for one additional case due to the exposure.
   • Formula:

Sample Cohort Study Data Table

Case Study Example: Children Living Near a Chemical Plant

Consider a cohort of 200 children divided into two groups: 100 children living near a chemical plant (exposed group) and 100 children not living near the plant (unexposed group). The goal is to assess if proximity to the plant (risk factor) increases the likelihood of developing asthma.

1. Calculate Relative Risk (RR)
   • Exposed Group Incidence Rate: 20100=0.2 ​=0.2
   • Unexposed Group Incidence Rate: =0.05 ​=0.05RR=0.20.05=4
   • RR = = 4 RR​=4
   • Interpretation: Children living near the chemical plant are 4 times more likely to develop asthma than those not living near the plant.
2. Calculate Attributable Risk (AR)
   • AR=0.2−0.05=0.15
   • Interpretation: Out of every 100 children exposed to the chemical plant, 15 cases of asthma are attributable to this exposure.
3. Calculate the Number Needed to Harm (NNH)
   • NNH =
   • Interpretation: For every 7 children exposed to the chemical plant, 1 additional case of asthma is expected due to the exposure.

Points to Remember

• Relative Risk (RR) assesses how much more likely one group is to develop a disease than another group.
• Attributable Risk (AR) evaluates how many more cases occur due to the exposure.
• Number Needed to Harm (NNH) calculates the number of individuals needed to be exposed for one additional case to be caused by the exposure.
• Neither RR nor AR indicates causation; it only indicates association.
• In interpreting these values, remember that cohort studies identify risk relationships, not causative mechanisms.

References

1. Porta, M. (Ed.). (2014). A Dictionary of Epidemiology (6th ed.). Oxford University Press.
2. Rothman, K. J., Greenland, S., & Lash, T. L. (2008). Modern Epidemiology (3rd ed.). Lippincott Williams & Wilkins.
3. Aschengrau, A., & Seage, G. R. (2013). Essentials of Epidemiology in Public Health (3rd ed.). Jones & Bartlett Learning.