Lecture 16: Measuring Disease Occurence Flashcards
Define epidemiology
The study of the occurrence and distribution of health-related events, states or processes in specified populations
What are the two measures of occurrence of disease?
- prevalence
- incidence
Incidence is one measure of occurrence. What are the two types of incidence?
- incidence proportion
- incidence rate
Define prevalence
The proportion of a population who have the disease at a point in time
Why do we care about the prevalence of disease?
Because it tells us about the burden of disease so we know where we can allocate resources
How can you calculate prevalence?
the number of people with a disease at a given point in time divided by the total number of people in the population at that point in time
What five things do we need to include when reporting prevalence?
- measure of occurrence
- exposure or outcome
- population
- time point
- value
How do you report prevalence of asthma among people taking POPH192 if the prevalence was 10% on the 10 Aug?
The prevalence of asthma in the POPH192 class on August 10th 2020 was 10%
What are some limitations of using prevalence?
- it is difficult to measure the development of disease
- it is influenced by the duration of disease
How does duration affect prevalence?
Because prevalence is looking at the number of people who have a disease in a given point in time, if the duration of the disease is longer, at one point in time, it may look like there are more people with the disease than if the duration was shorter
Define incidence
the occurrence of new cases of an outcome in a population during a specified period of follow up
Define incidene proportion
The proportion of an outcome-free population that develops the outcome of interest in a specified time period
How do you calculate incidence proportion?
the number of people who develop the disease in a specified period divided by the 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 the study?
- they already have the condition
- the condition is something that they cannot develop
What five things do we need to include when reporting incidence proportion?
- measure of occurrence
- exposure or outcome
- population
- time point
- value
How do you report incidence proportion of lower back pain among among nurses if the proportion was 35% in 12 months?
the incidence proportion of lower back pain in nurses in 12 months was 35%
Why should we use the incidence proportion?
the incidence proportion tells us about the risk of developing an outcome
What are two limitations of using incidence proportion?
- it assumes a closed population (does not account for people coming or going)
- it is highly dependent on the time period (longer time period means a higher incidence proportion)
Define incidence rate
the rate at which new cases of the outcome of interest occur at a population
How do you calculate the incidence rate?
the number people who develop the disease in a specified period divided by the number of person years at risk of developing the disease
How do you calculate the number of person years at risk?
it is the sum of everyone in the population’s time at risk of becoming a case
Why might some people stop being at risk when calculating person-years at risk?
- they become a case
- they are lost to follow up
- the follow up time ends
What four things need to be included when reporting incidence rate?
- measure of occurrence
- outcome
- population
- value
How do you report incidence rate of glandular fever in the class if the rate was 0.5 case per person year?
The incidence rate of glandular fever in the class was 50 per 100 person-years
What are two limitations of the incidence rate?
- there can be no person-time information available
2. it can complex to calculate
How does prevalence relate to incidence and duration?
Prevalence approximates incidence multiplied by the disease duration (although this can’t be used as an actual formula) so changes to the incidence and duration can affect the prevalence
When should we age standardise?
when the age structures between two populations differs and when the disease risk varies by age
Why do we need to age standardise?
If the outcome of interest is an age related outcome and the age structures differ between the populations then if we don’t age standardise then the difference in occurrence we see could just be because of the difference in age
