Lecture 16 Flashcards
3 measures of occurrence
- prevalence
- incidence proportion
- incidence rate
Definition of 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
- impact among different groups
- trends over time
Two measures of occurrence
What is prevalence
The proportion of a population who have the disease at a point in time
Prevelance
Why do we measure prevalence ?
• Burden of disease
• Resource allocation
Calculation of prevelance in a population
Must have all 5 to interpret findings in prevalence reporting
Limitiations when measuring prevelance
- 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
16
C
Incidence proportion is sometimes referred to as
cumulative incidence
The influence of duration of the disease on prevalence
What is the incidence
The occurrence of new cases of an outcome in a population during a specific period of follow-up
- incidence proportion (IP)
- incidence rate (IR)
Key difference between IP and IR
what we use as the denominator
What is the Incidence proportion
The proportion of an outcome-free population that develops the outcome of interest in a specified time period
Incidence proportion calculation
Why might people not be considered ‘at risk’ at the start of a study? - for an incidence prportiaon calculation ?
• They already have the condition
• The condition is something that they cannot develop
5 things needed doe incidence proportion reporting
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)
What is the incidence rate?
The rate at which new cases of the outcome of interest occur in a population
(How quickly are new cases of the condition developing in the population?)
Incidence rate calculation
person-years at risk
Sum of everyone in the population’s time at risk of becoming a case
What stops someone being ‘at risk’ - incidence rate calculation
• They become a case
• They are lost to follow-up
( e.g. die, move away, no longer take part)
• Follow-up time ends
4 things needed when measuring incidence rate
Incidence rate - limitations
Person-time not available
Complex to calculate
Prevalence, incidence and duration relationship
P~I x D
Changes to incidence and duration can affect disease prevalence
(Don’t use as equation)
Comparing populations - age standardisation - WHO standard population
Do the age structures differ?
Does the disease risk vary by age?
Age standardisation
When do we need to do age standardisation ?
- age structures differ
- disease risk varies by age
