Cohort studies

• What is natural history refer to?

  How a disease evolves over time and to the factors that shape its evolution
• What study investigates the natural history of a disease?

  Cohort studies
• What are the 3 components of cohort studies?

  Follow a defines group of participants

  Over a period of time

  Record the incidence of events of interest
• What are the 2 main types of cohort studies?

  Population cohorts

  Clinical cohorts
• What are population cohorts ?

  Follow up of population groups to document incidence of diseases or disorders and their risk factors.
• What do population cohorts tell us?

  How the disease develops in healthy people
• What are clinical cohorts?

  Follow up of patient groups to document natural history and prognosis of a disease or a disorder
• What do clinical cohorts tell us?

  Tell us about the natural history and prognosis of a disease
• What do population cohort studies provide information on?

  The incidence of health problems
• General population studies often take which individuals?

  Take the population of a defined geographic region
• How are population cohort conducted?

  Identify a population group of healthy individuals (not having disease).

  Measure characteristics at baseline (classify into groups depending on exposure status of risk factors)

  Follow up over time to assess defined end points to determine incidences.
• What is survival time?

  The time in which you track a person (time of their study) individually. eg. when they start to when they drop out or have the outcome/disease
• What curve is used in cohort studies to track peoples survival times ?

  Kaplan meier (KM) curve
• Why are survival analysis important?


  Special statistical methods required to account for variable follow up
• What do KM curves show?

  Whether or not events (outcomes) happen and how long it takes for them to happen
• What do the steps on the KM curve describe?

  Represent outcome occurring (or people dropping out).
• What happens to the steps as time goes on and why?

  Bigger drops at the end to indicate there are few patients left.
• What is the middle of the curve called?

  Median survival time
• Cohort studies provide strong evidence that the association between risk factor and outcome is what? why?

  Causal. This is because the risk factors are measured before the disease occurs
• What do clinical cohort studies study?

  Natural history and prognosis
• What is a cohort?


  group of participants with a defined diagnosis or condition
• When are patients followed up in clinical cohort studies?

  Followed from a defined point in the natural history of the disease or condition
• What do cohort studies record?

  Incidence of clinical endpoints and factors which affect the incidence
• What are the strengths of cohort studies?

  Often study multiple different endpoints

  They record incidence which is important info for healthcare planning

  Less biased info on risk factors because teh risk factor is measured before the disease occurs,
• What is incidence?


  The rate of occurrence of new cases per unit time per unit population.
• What does incidence indicate?

  Risk of contracting disease
• What are the weaknesses of cohort studies?

  Collecting data requires follow up (long duration)

  Even for common diseases, a lotg of follow up is needed to detect a worthwhile number of cases.

  Need large sample size (non-response, migration, loss to follow up and selection biases)

  Expensive

  Impractical for diseases with a long pre-clinical phase

  Impractical for rare diseases
• What do population cohort studies study?

  Incidence of disease to look for risk factors
• What do clinical cohort studies examine?

  Natural history and prognosis of a disease looking for factors that affect prognosis and possible mechanisms by which they work,