M09.01.013 Cohort study design

Written by

in

Learning Objective

To understand how cohort studies measure and interpret associations between exposure and disease using Relative Risk (RR), Attributable Risk (AR), and Number Needed to Harm (NNH).

Cohort Studies: Measuring Risk

A cohort study follows a group of individuals over time to evaluate whether exposure to a risk factor affects the development of a disease. Cohort studies allow estimation of both relative and absolute risk, informing clinical and public health decisions.

 

Relative Risk (RR)

  • Measures the strength of the association between exposure and disease.
  • Question: How much more likely is the exposed group to develop the disease compared to the unexposed group?
  • Formula:
RR = \frac{\text{Incidence in Exposed Group}}{\text{Incidence in Unexposed Group}}

Example: In a cohort of 100 children exposed to a chemical plant, 20 develop asthma. In 100 unexposed children, 5 develop asthma.
RR = 0.20 / 0.05 = 4 → Exposed children are 4 times more likely to develop asthma.

Attributable Risk (AR)

  • Measures the absolute difference in disease incidence between exposed and unexposed groups.
  • Question: How many additional cases occur due to the exposure?
  • Formula:
AR = \text{Incidence in Exposed Group} - \text{Incidence in Unexposed Group}

Example: Continuing the same cohort, AR = 0.20 – 0.05 = 0.15 → Out of 100 exposed children, 15 additional asthma cases are attributable to exposure.

3. Number Needed to Harm (NNH)

  • The number of individuals who must be exposed for one additional case to occur due to the exposure.
  • Formula:
NNH = \frac{1}{AR}

Example: Using AR = 0.15, NNH = 1 / 0.15 ≈ 6.66 ≈ 7 → For every 7 children exposed, 1 extra case of asthma occurs.

Sample Cohort Study Data

Disease Present Disease Absent
Exposed A = 60 B = 240
Unexposed C = 60 D = 540

Case Example: Children Living Near a Chemical Plant

  • Cohort: 200 children
  • Exposed Group: 100 children living near the chemical plant
  • Unexposed Group: 100 children living away
  • Outcome: Incidence of asthma
Group Total Asthma Cases Incidence
Exposed 100 20 0.20
Unexposed 100 5 0.05

Step 1: Calculate Relative Risk (RR)

\text{Incidence in Exposed} = \frac{20}{100} = 0.20 \text{Incidence in Unexposed} = \frac{5}{100} = 0.05 RR = \frac{0.20}{0.05} = 4

Interpretation: Children living near the plant are 4 times more likely to develop asthma.

Step 2: Calculate Attributable Risk (AR)

AR = 0.20 - 0.05 = 0.15

Interpretation: Out of 100 exposed children, 15 asthma cases are attributable to the exposure.

Step 3: Calculate Number Needed to Harm (NNH)

NNH = \frac{1}{0.15} \approx 6.66 \approx 7

Interpretation: For every 7 children exposed, 1 extra case of asthma occurs due to exposure.

Key Points

  • RR: Measures relative likelihood of disease.
  • AR: Measures absolute excess risk due to exposure.
  • NNH: Number of exposures needed to produce one extra case.
  • Cohort studies indicate association, not causation.
  • Useful for risk assessment and public health planning.

Activity:

Discover more from mymedschool.org

Subscribe to get the latest posts sent to your email.

Medical courses around the world

More posts