4.13

Question 1

Prevalence

Answer 1

Number of existing cases of an outcome of interest in a defined population, at one point (or period) in time.

Expressed as proportion or percentage.

Question 2

Prevalence

Answer 2

Number of existing cases of an outcome of interest in a defined population, at one point (or period) in time.
Expressed as proportion or percentage.

Question 3

Incidence

Answer 3

Number of new cases of an outcome of interest arising from a defined population, during a time interval.
Expressed as a rate i.e. denominator includes a time component.
ONLY from longitudinal studies.

Question 4

Risk

Answer 4

Probability of disease occurring in a disease-free population during a specified time period.

Risk = n/P

n = new cases in a defined period, P = population at risk.

Question 5

Rate

Answer 5

Rate – probability of disease occurring in a disease-free population during the sum of individual follow up periods.

Rate = n/total(person-time of follow up)

n = new cases in a defined period, total(person-time of follow up) = totals of time of follow up for all persons in study.

Question 6

Absolute risk/rate

Answer 6

Isolated measurement of risk/rate.

Examples:
5 strokes/10000 men per year.
10 MIs/1000 person-years.

NO indication of association with exposure i.e. NO indication of causes of strokes or MIs.

Question 7

Relative risk

Answer 7

Synonyms – risk ratio, rate ratio.

RR = Re/Ru

Indicates relative magnitude of change in risk/rate of outcome, associated with exposure.

Question 8

Attributable risk

Answer 8

Synonyms – risk difference, rate difference.

AR = Re – Ru

Indicates absolute magnitude of change in risk/rate of outcome, associated with exposure.

Question 9

Attributable risk percent

Answer 9

Proportion of incident disease among exposed people that is due to exposure.

AR% = [(Re – Ru)/Re] x 100

Interpretation e.g. 50% of incident disease among people who are exposed is due to the exposure.

Question 10

Population attributable risk

Answer 10

PAR = Rt - Ru

Rt = risk/rate in whole population (BOTH exposed and unexposed).

Indicates additional or excess risk/rate of the outcome in the population, due to the exposure.

Question 11

Population attributable risk percent

Answer 11

Synonym – preventable fraction.

PAR% = [(Rt – Ru)/Rt] x 100

Proportion of incident disease among whole population that is due to exposure.

Interpretation e.g. 38% of the incident disease among the whole population is due to the exposure (and 62% due to something else).

Question 12

Incidence

Answer 12

Number of new cases of an outcome of interest arising from a defined population, during a time interval.

Expressed as a rate i.e. denominator includes a time component.

ONLY from longitudinal studies.