Epidemiology

Question 1

Prevalence

Answer 1

Proportion of animals in a population that has the disease of interest at a specific time

Number of cases at time t / number of individuals at time t

Requires cross sectional studies

Question 2

Cumulative incidence

Answer 2

The proportion of healthy individuals that develop disease of a specified time period out of those at risk

Number of new cases during time period / population at risk during same period

Requires clear case definition + surveillance programme

Question 3

Relative risk

Answer 3

Risk of disease in exposed animals / risk of disease in unexposed animals

Question 4

Odds

Answer 4

Probability of event occurring / probability of event not occurring

Question 5

Odds ratio

Answer 5

Odds of disease in exposed animals / odds of disease in unexposed animals

Question 6

Attributable risk

Answer 6

Risk of disease in exposed animals - risk of disease in unexposed animals

Question 7

Attributable fraction

Answer 7

(risk among exposed - risk among unexposed) / risk among exposed

Question 8

Koch’s Postulates

Answer 8

An agent must follow this criteria to be considered an agent of disease:
- It has to be present in every case of the disease
- It has to isolated and grown in pure culture
- It has to cause specific disease when inoculated into a susceptible animal and can then be recovered from the animal and identified.

Question 9

Evan’s postulates

Answer 9

Prevalence of disease should be higher in those exposed than unexposed.

Exposure should be more common in those with the disease than in those without the disease

The disease should follow exposure to the age along a logical biologic gradient from mild to severe

Temporarily, the disease should follow exposure to the putative agent

Question 10

Association

Answer 10

A state in which two attributes occur together either more or less than expected by chance

Question 11

Criteria for establishing causal association

Answer 11

The effect has to occur after the cause

Larger the association, the more likely that it is causal

Greater exposure should generally lead to greater incidence

Consistent findings observed

Compatibly with biological mechanism

Question 12

Non causal associations may arise due to:

Answer 12

Chance
Reverse causation
Bias
Confounding

Question 13

Selective bias

Answer 13

When selected individuals are systematically different to those not selected for study

Question 14

Spectrum bias

Answer 14

Cases with certain signs may be more likely to be detected

Question 15

Incidence prevalence bias

Answer 15

Results from the inclusion of prevalent cases in a study

Question 16

Interpretive bias

Answer 16

When different emphasis is given to different evidence when results are evaluated

Question 17

Confounding

Answer 17

When the apparent association between a study factor and an outcome is distorted by the effect of a third variable which is associated with both the outcome and the factor

Question 18

Requirements for a variable to be confounding

Answer 18

Must be associated with the outcome

Must be associated with the exposure factor

Mustn’t be an intermediate step in a causal pathway

Question 19

Case study

Answer 19

What are the features of this disease?
Most useful:
- When reporting a new disease/therapy
- In the formulation of hypotheses for further study

Question 20

Cross sectional study

Answer 20

Currently, how common is this disease?
Provide information on:
- Prevalence of disease/exposure
- Associations between the disease and study factors

Question 21

Case-control studies

Answer 21

Individuals are classified by their disease status/outcome
- Cases
- Controls
Then past exposure to risk factor of interest is measured for each group
Analysis then aims to quantify the alteration in risk associated with each factor