Basic Measures of Health

Question 1

What is health?

Answer 1

Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.

Question 2

What is prevalence?

Answer 2

The proportion of individuals in a population who have a disease or an attribute at a specified point in time.

Prevalence = No. with disease at given point in time / total no. in population

Question 3

What is incidence?

Answer 3

Number of NEW health-related events in a (defined) population within a specified period of time.

Question 4

What is cumulative incidence?

Answer 4

CI = No. of people who develop disease in a specified time period / No at risk of getting disease at start of time period.
Expresses as percentage.
It is a measure of disease frequency during a period of time.

Question 5

What is incidence rate?

Answer 5

IR= No. who develop disease / No person-time when people were at risk of getting disease.
Expressed in person-years.
It is a measure of the risk of developing some new condition within a specified period of time.

Question 6

Explain person-years.

Answer 6

The product of the amount of time X the number of members of a population who have been affected by a certain condition.

Question 7

Fifty children living near a nuclear power plant who were at risk of developing leukaemia at the start of the study were followed up for 2 years and 2 developed leukaemia.
What is the cumulative incidence of leukaemia in the study?

Answer 7

CI= 2/50 = 0.04 = 4% of children developed leukemia in the 2 year period

Question 8

Followed up 5 individuals at risk of Upper Respiratory Tract Infection over a 12 month period. Two people developed URTI. With 39 person-months.

What is the Incidence Rate of URTI per person-months?
What is the Incidence Rate of URTI per person-years?

Answer 8

IR= 2/39 = 0.051 cases of URTI per person-month.

0.051 * 12 = 0.62 cases of URTI per person-year.

Question 9

What relates prevalence and incidence?

Answer 9

Prevalence = IR * average duration of disease.

This assumes balance between entering (birth) and exiting (death) population. E.g. HIV incidence drops every year but prevalence increases due to not having a stationary population.

Question 10

What increases prevalence?

Answer 10

Longer duration
Increase incidence
In-migration of unhealthy
Out-migration of healthy
Improved diagnostics

Question 11

What decreases prevalence?

Answer 11

Shorter duration
High fatality
Less incidence
Out-migration of unhealthy
In-migration of healthy

Question 12

Explain case definition.

Answer 12

Important to be specific and standardized – who is healthy and who is not.