LECTURE 16: measuring disease occurrence Flashcards
why measure disease occurrence in populations?
- health status
- impact among different groups
- trends over time
what are the 2 measures of occurrence
- prevalence
2. incidence
2 types of incidence
- incidence proportion
2. incidence rate
what is prevalence
the proportion of a population who HAVE the disease at a point in time
how to calculate prevalence
number of people with the disease at a given point in time / total number of people in the population at that point in time
reporting of prevalence
measure of occurrence exposure or outcome population time point value
prevalence limitations
- difficult to assess the development of disease
2. is influenced by the duration of the disease
what is incidence
the occurrence of new cases of an outcome in a population during a specific period of follow-up
what is the key difference between IP and IR
what we use as the denominator
what is incidence proportion
The proportion of an outcome-free population that develops the
outcome of interest in a specified time period
how to calculate incidence proportion
number of people who develop the disease in a specific period (new cases) / 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 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 incidence rate
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
how to go from person-months to person-years
divide by 12
incidence rate reporting
measure of occurrence
outcome
population
value
incidence rate limitations
- person-time no available
2. complex to calculate
what does prevalence approximate?
incidence x average disease duration
changes to incidence and duration can affect…
disease prevalence
comparing populations
- Do the age structures differ?
2. Does the disease risk vary by age?
2 aspects as to when we use age standardisation
age structures differ and disease risk varies by age
