Measuring Health in Populations - Lecture Sixteen

Question 1

Why measure disease occurrence in populations?

Answer 1

Health status
Trends over time
Impact among different groups

Question 2

Prevalence - what?

Answer 2

The proportion of a population who have the disease at a point in time

Question 3

Prevalence - why?

Answer 3

Burden of disease

Resource allocation

Question 4

Prevalence - calculation

Answer 4

Number of people with the disease at a given point in time / Total number of people in the population at that point in time

Question 5

Prevalence - reporting

Answer 5

Measure of occurrence
Exposure or outcome
Population
Time point
Value

Question 6

Prevalence limitation

Answer 6

Difficult to assess the development of disease

Is influenced by the duration of the disease

Question 7

Incidence

Answer 7

The occurrence of new cases of an outcome in a population during a specific period of follow-up

Question 8

Incidence Proportion

Answer 8

The proportion of an outcome-free population that develops the outcome of interest in a specified time period

Question 9

Incidence proportion calculation

Answer 9

Number of people who develop the disease in a specified period / Number of people at risk of developing the disease at the start of the period

Question 10

What might people not be considered ‘at risk’ at the start of a study?

Answer 10

They already have the condition

The condition is something that they cannot develop

Question 11

Population at risk

Answer 11

New cases during the time period / Population at risk at the beginning of the time period

Question 12

Incidence proportion reporting

Answer 12

Measure of occurrence 
Outcome
Population
Time period
Value

Question 13

Incidence proportion limitations

Answer 13

Assumes a ‘closed’ population (does not account for people coming or going) e.g. someone might die or move away
Highly dependent on the time period (longer time period = higher incidence proportion)

Question 14

Incidence rate

Answer 14

the rate at which new cases of the outcome of interest occur in a population

Question 15

Incidence rate what and why

Answer 15

How quickly are new cases of the condition developing in the population?

Question 16

Incidence rate calculation

Answer 16

Number of people who develop the disease in a specified period / Number o person-years at risk of developing the disease

Question 17

Why might someone stop being ‘at risk’

Answer 17

They become a case
They are lost to follow-up, e.g. die, move away, no longer take part
Follow-up time ends

Question 18

Incidence rate reporting

Answer 18

Measure of occurrence
Outcome
Population
Value

Question 19

Incidence rate limitations

Answer 19

Person-time not available

Complex to calculate

Question 20

Prevalence, incidence and duration

Answer 20

P ~ I x D, Prevalence approximates incidence x average disease duration
Changes to incidence and duration can affect disease prevalence - a patient might recover faster (shorter duration & lower prevalence) but patient might live longer with the disease (longer duration & more prevalence)