PUBLIC HEALTH EXTRAS

Question 1

EPIDEMIOLOGY
Define epidemiology. What factors are considered when measuring epidemiology of a disease?

Answer 1

The study of the frequency, distribution + determinants of disease + health-related states in populations in order to prevent + control disease.
- Time, place, person (age, gender, class, ethnicity).

Question 2

EPIDEMIOLOGY
Define incidence.

Answer 2

The number of new cases of a disease that develop in a population (e.g. per 100,000) in a given time frame (e.g. per year).

Question 3

EPIDEMIOLOGY
Define prevalence.

Answer 3

The total # of people in a population found to have a disease at a point in time.
- Number of existing cases/population/points in time.

Question 4

EPIDEMIOLOGY
What is meant by person-time?

Answer 4

The measure of time at risk i.e. time from entry to a study to:
- Disease onset.
- Loss to follow up.
- End of study.
It is the sum of each individual’s time at risk (i.e. length of time they were followed up in the study).

Question 5

EPIDEMIOLOGY
How do you calculate incidence rate calculations?

Answer 5

of persons who have become cases in a given time period ÷ total person-time at risk during that period.

• Person-time is the denominator in them.

Question 6

EPIDEMIOLOGY
What is the denominator in cumulative incidence calculations?

Answer 6

Number of disease-free at start of study.

Question 7

EPIDEMIOLOGY
What is meant by:
i) an independent variable?
ii) a dependent variable?

Answer 7

i) A variable that can be altered in a study.
ii) A variable that is dependent on the independent variables or one that cannot be altered.

Question 8

EPIDEMIOLOGY
What is meant by absolute risk?

Answer 8

Gives a feel for actual numbers involved i.e. it has units.
- E.g. deaths/1000 population.

Question 9

EPIDEMIOLOGY
What is meant by relative risk?

Answer 9

Ratio of risk of disease in the exposed to the risk in the unexposed i.e. no units.
- It tells us about the strength of association between a risk factor + a disease.

Question 10

EPIDEMIOLOGY
How do you calculate relative risk?

Answer 10

Incidence in exposed ÷ incidence in unexposed.

Question 11

EPIDEMIOLOGY
What is meant by attributable risk?

Answer 11

The rate of disease in the exposed that may be attributed to the exposure.
- Attributable risk is a type of absolute risk (absolute excess risk).
- It’s about the size of effect in absolute terms i.e. gives a feel for the public health impact (if causality assumed).

Question 12

EPIDEMIOLOGY
How do you calculate attributable risk?

Answer 12

Incidence in exposed – incidence in unexposed.

Question 13

EPIDEMIOLOGY
What is meant by relative risk reduction?

Answer 13

The reduction in rate of the outcome in the intervention group relative to the control group.

Question 14

EPIDEMIOLOGY
How do you calculate relative risk reduction?

Answer 14

(Incidence in unexposed – incidence in exposed) ÷ incidence in unexposed.
- (ARR ÷ incidence in unexposed).

Question 15

EPIDEMIOLOGY
What is meant by absolute risk reduction?

Answer 15

The absolute difference in the rates of events between the 2 groups. Gives an indication of the baseline risk + the intervention effect.

Question 16

EPIDEMIOLOGY
Give an example of absolute risk reduction?

Answer 16

Assuming exposed means they have had a particular intervention (such as giving statins to people with hypercholesterolaemia) + then a control group who do not have statins + seeing how many in each group have a heart attack to see if the intervention of statins is effective.

Question 17

EPIDEMIOLOGY
How do you calculate absolute risk reduction?

Answer 17

Incidence in unexposed – incidence in exposed.

Question 18

EPIDEMIOLOGY
What is meant by odds?

Answer 18

The odds of an event is the ratio of the probability of an occurrence compared to the probability of a non-occurrence.

Question 19

EPIDEMIOLOGY
How do you calculate odds?

Answer 19

Probability ÷ (1 – probability).

Question 20

EPIDEMIOLOGY
What is meant by odds ratio?

Answer 20

The ratio of odds for the exposed group to the odds for the non-exposed groups.
- Or can be interpreted as a relative risk when the event is rare.

Question 21

EPIDEMIOLOGY
When would odds ratio be used?

Answer 21

For case control studies as it’s not possible to calculate the relative risk.
- For cross-sectional + cohort studies – both can be derived but odds ratio is used if it’s not clear which is the independent/dependent variable.

Question 22

EPIDEMIOLOGY
How do you calculate odds ratio?

Answer 22

ODDS EXPOSED / ODDS UNEXPOSED

(P exposed ÷ [1 – P exposed]) ÷ (P unexposed ÷ [1 – P unexposed])

Question 23

EPIDEMIOLOGY
What is meant by number needed to treat?

Answer 23

The number of patients that need to be treated in order to prevent one bad outcome.

Question 24

EPIDEMIOLOGY
How do you calculate NNT?

Answer 24

1 ÷ ARR (risk in unexposed – risk in exposed)