Measuring disease occurrence Flashcards
What are the measures of occurrence?
Prevalence, Incidence proportion (sometimes called cumulative incidence) and incidence rate
What is prevalence, why do we use it and what are its limitations?
The proportion of a population who have the disease at a point in time
Use it for burden of disease and resource allocation
Limitations:
- Difficult to assess the development of diseases
- Is influenced by the duration of the disease
How do you 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
What are the five things to include in reporting?
- Measure of occurrence (prevalence, incidence proportion or incidence rate)
- Exposure or outcome (what you are measuring)
- Population
- Time point
- Value
Note: incidence rate doesn’t include a time point in the reporting
A longer duration has what affect on prevalence?
Prevalence gets higher
What is incidence proportion (IP), why might people not be considered at risk at the start of the study and what are the limitations ?
The proportion of an outcome-fee population that develops the outcome of interest in a specified time period
May not be considered at risk because:
- they already have the condition
- the condition is something that they cannot develop
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)
How do you calculate incidence proportion?
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
What is the incidence rate, why might some people stop being ‘at risk’ and what are the limitations?
The rate at which new cases of the outcome of interest occur in a population (how quickly are new cases developing in the pop)
May stop being ‘at risk’ because:
- they become a case
- they are lost to follow-up
- follow-up time ends
Limitations:
- Person-time not available
- complex to calculate
How do you calculate person time?
Add up the time from ALL the people at risk that they did NOT have the illness (make sure it is in years! divide my 12 if its in months)
- It is the sum of everyone in the population’s time at risk of becoming a case
How do you calculate incidence rate?
Number of people who develop the disease in a specified period / Number of person-years at risk of developing the disease
How do incidence and duration affect prevalence?
Prevalence approximates incidence x average disease duration.
Changes to incidence and duration can affect disease prevalence.
What do we have to take into account when comparing populations?
- If the age structures differ
- If he disease risk varies by age
When do we have to use age standardisation?
If the age structures differ AND the disease risk varies by age
