Lecture 16: Measuring disease occurrence Flashcards
Define Epidemiology
“The study of the occurrence and distribution of health-related
events, states or processes in specified populations…
Why measure disease occurrence in
populations?
Health status
Trends over time
Impact among different groups
What is prevalence?
The proportion of a population who have the disease at a point in
time
Prevalence - calculation
Number of people with the disease at a given point in time/Total number of people in the population at that point in time
Prevalence - reporting
Measure of occurrence
Exposure or outcome
Population
Time point
Value
Prevalence - limitations
- Difficult to assess the development of disease
- Is influenced by the duration of the disease
Which of the following would be the most appropriate numerator
for determining the prevalence of measles on 31 December
2022?
a. The number of people diagnosed with measles during 2022
b. The total number of people in the population on 31 December 2022
c. The number of people with measles on 31 December 2022
C
Incidence proportion
The proportion of an outcome-free population that develops the
outcome of interest in a specified time period
Incidence proportion - calculation
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
Why might people not be considered ‘at risk’ at the start of a study?
They already have the condition
The condition is something that they cannot develop
Incidence proportion - reporting
Measure of occurrence
Outcome
Population
Time period
Value
Incidence proportion - limitations
Assumes a ‘closed’ population
(does not account for people coming or going)
Highly dependent on the time period
(longer time period = higher incidence proportion)
Incidence rate – what and why
The rate at which new cases of the outcome of interest occur in a population
Incidence rate - calculation
Number of people who develop the disease in a specified period/Number of person-years at risk of developing the disease
Why might someone stop being ‘at risk’?
They become a case
They are lost to follow-up
(e.g. die, move away, no longer take part)
Follow-up time ends
