Maths for Clinicians 9

• What is a cohort study?

  Type of longitudinal study

  Follows group of people (initially disease free) over time to determine incidence of specific diseases. Sees how exposure affects their outcomes.

  Strongest type of observational study
• What is clinical medicine concerned with?

  Cases of diseases for individual patient
• What is epidemiology concerned with?

  Disease rates in populations

  Identify risk factors that cause certain populations to be at greater risk of disease.
• How do you calculate prevalence of disease?


  :

  
• How do you calculate risk of disease?


  :

  
• How do you calculate the incidence rate?


  :

  
• How do you calculate the risk difference?

  Risk in exposed - Risk in unexposed
• EXAMPLE:

  
• WORKINGS OUT:

  
• What are the strengths of cohort studies?

  • Can draw causal conclusions as exposure is measured before onset of disease
  • Can look at multiple exposures & outcomes
  • Rare exposures can be examined
  • Can measure incidence & prevalence
• What are the weaknesses of cohort studies?

  • Costly
  • Time-consuming
  • Bias due to loss of follow up
  • Prone to confounding
  • Being in study may alter participant behaviour
  • Poor choice for study of rare disease (small cohort)
  • Participants may move between exposure category overtime
  • Classification of exposure or outcome status can be affected by changes in diagnostic procedures
• What is risk?

  • Probability individual will develop disease by specific time point
  • Proportion
  • Cumulative over time
• What is rate?

  • How rapidly new events occur per unit time (e.g, per 1000 person years)
  • Can handle variable follow-up times & attrition