Epidemiology

Question 1

What is mortality rate?

Answer 1

You need a denominator and timeframe to define it

Question 2

What is incidence?

Answer 2

Number of new cases

Useful for identifying causes of diseases

Question 3

What is prevalence?

Answer 3

Proportion of population that has disease
Useful for planning services

Types:
Point
- number of people with outcome at a point in time/total number of people in the group x100

Period - number of people with outcome during a time period/average number of people in the group x100

Question 4

How do you calculate effect estimates?

Answer 4

Risk:
- number of outcomes in a group/number of people in the group x100

Relative risk/Rish ratio:
- risk in exposed/risk in unexposed

Relative risk reduction:
- (1 - relative risk) x100

Absolute risk reduction:
- Risk in unexposed - Risk in exposed

Number needed to treat:
- 1/absolute risk reduction

Question 5

Confidence intervals?

Answer 5

The higher the line, the more plausible the value

Represents range of plausible values

Question 6

Describe the hierarchy of evidence

Answer 6

Top to bottom:

• Systematic reviews and meta-analysis
• Randomised controlled trial
• Controlled study
• Cohort study/case-controlled study
• Cross-sectional study
• Background information/expert opinion

Question 7

What is a cross-sectional study?

Answer 7

Sampling a population, estimating the proportion
Use data to describe prevalence
(looks at the prevalence; past)

Question 8

What is a case-control study?

Answer 8

Select cases with an outcome
Select controls without the outcome
Explore EXPOSURES in cases and controls
Compare exposures in cases and controls
Identify association
(looks at the cause; past)

Question 9

What is a cohort study?

Answer 9

Select people without an outcome
Classify according to an exposure
Follow up
Compare RISK of disease in exposed and unexposed

Can be prospective (future) or retrospective (past)

(looks at the cause, prognosis, incidence)

Question 10

What is a randomised controlled trial?

Answer 10

Random allocation
Compare RISK of outcome in intervention and control groups
(looks at the treatment effect; future)

Question 11

What is confounding?

Answer 11

True relationship is confused by a third factor

Question 12

What is bias?

Answer 12

Systematic error

• what and how data are controlled
• how data are analysed, interpreted and reported

Bias leads to wrong conclusions concerning effectiveness and causation

Top of the hierarchy of evidence is less prone to bias

Question 13

What are the 9 criteria for causality? (for something to be able to cause an effect)

Answer 13

Strength
- better if strong

Consistency
- better if observed in different studies/sub-groups

Specificity
- disease is associated with one specific factor

Temporality
- a casual link is more likely if exposure to the cause is shown to precede the outcome

Biological gradient
- different level of exposure lead to different risk of acquiring an outcome

Plausibility
- biologically plausible mechanism

Coherence
- Conforms with current knowledge

Experiment
- If removal or prevention leads to a reduced or non-existent risk of acquiring the outcome

Analogy
- if an analogy exists with other diseases, species or settings