(4) The role of cohort studies

Question 1

Definition of a prospective cohort study

Answer 1

An observational study where a population is studied for the presence of a fixed or modifiable exposure, which is thought to be an aetiology of a condition (e.g. CHD), prior to the onset of a condition. The entire population is followed up in time, and the incidence of disease in exposed individuals is compared with the incidence in those not exposed.

Question 2

What are the modifiable/nonmodifiable risk factors for CHD?

Answer 2

Modifiable: 
- Physical activity
- Body size
- Diabetes
- Blood Pressure
- Blood lipids
- Smoking
Non-modifiable:
- Age
- Gender
- Genes
- family history

Question 3

What are confounding variables?

Answer 3

A confounding variable is an outside influence that changes the effect of a dependent and independent variable.

Question 4

What is the direct method of standardising?

Answer 4

Rates from each population studied applied to a standard population to give age and/or sex adjusted mortality (or morbidity) rates

Question 5

What is the indirect method of standardising?

Answer 5

Rates from a standard population applied to each population of interest to give standardized mortality (morbidity) ratios (so called SMRs)

Question 6

Advantages of cohort studies

Answer 6

• Evidence of exposure does not depend on memory (except in historical cohorts)
• Exposure is measured before disease onset, reducing potential for bias, and allowing causality to be assumed
• Multiple outcomes (diseases) can be studied for any one exposure
• Incidence of disease can be measured in the exposed and unexposed -groups
• Potential for nested case control studies

Question 7

What is a nested case control study?

Answer 7

A cohort study with a casecontrol study nested within it

Question 8

Disadvantages of a cohort study

Answer 8

• Slow, expensive, administratively difficult and complex
• Needs large numbers and a long time
• Collection of data may alter behaviour
• Problems keeping standards
• Changes in exposure and diagnostic criteria with time
• Ascertainment of outcome status may be influenced by knowledge of exposure
• Losses to follow up may introduce selection bias (i.e. those left different health behaviour)