Lecture 16 - Measuring Disease Occurence Flashcards
Why measure disease occurrence in
populations?
Health status
Trends over time
Impact among different groups
Measures of occurrence
Prevalence
Incidence
- incidence proportion (cumulative incidence)
- incidence rate
What is prevalence
proportion of a population who have the disease at a point in time
Point prevalence
Prevalence why
Burden of disease
Resource allocation
Prevalence calculation
Number of people with disease at given time / total number of people in that population in given time
Prevalence reporting
Measure of occurrence - prevalence Exposure of outcome - of disability Population - in people in NZ Time point - in 2013 Value - 23.8%
Prevalence - limitations
- Difficult to assess the development of disease
2. Is influenced by the duration of the disease
Incidence
occurrence of new cases of an outcome in a population during a specific period of follow-up
IP (Incidence proportion)
IR (Incidence rate)
Difference is what we use as the denominator
Incidence proportion
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
New cases / population at risk
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
New cases during the time period / population at risk at the beginning of the time period
Incidence proportion reporting
Measure of occurrence - incidence proportion Outcome - of low back pain Population- in nurses Time period - in 12 months Value - was 35%
Incidence proportion why
Risk (average)
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
*How quickly are new cases of the condition developing in the 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
Develop disease / person years at risk of developing disease
Multiply by 100
Incidence rate reporting
Measure of occurrence - incidence rate
Outcome - glandular fever
Population - in class
Value - 50 per 100 person-years
Incidence rate limitations
Person-time not available
Complex to calculate
Brief recap what is prevalence
Existing cases - distribution, burden
Influenced by duration, does not tell us about development
Brief recaps
What is incidence proportion?
New cases - risk
Doesn’t account for time-at-risk
Increases if time increases
Brief recap
What is incidence rate?
New cases - speed
Needs person-time - not always available, complex
Prevalence of gout in NZ
Shows burden of disease among different groups of a population
Higher in males, pacific, maori than european and females
Prevalence aspects
Disease Health condition Exposure - Possible determinant of disease - Eg smoking
Prevalence - reporting
Measure of occurrence Exposure or outcome Population Time point Value
Measure of occurrence
incidence rate
incidence proportion
prevalence
Exposure or outcome
health condition
Population
people in nz
what is incidence proportion?
People who develop the disease in specific time period (new cases of disease) - numerator
Denominator - people at risk of developing the disease at the start of the period
New cases / population at risk
Why incidence repost
Risk
Shows who have the highest risk
Average risk in that group
Incidence proportion - reporting
Measure of occurrence Outcome Population Time period Value
Treatment might mean people recover faster
Disease shorter duration
Low prevalence
Treatment might mean people may live longer with the disease
Increase duration
Increase prevalence
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?
rate at which new cases of the outcome of interest occur in a population
why use incidence rate?
How quickly are new cases of the condition developing in the 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
person-years
Sum of everyone in the population’s time at risk of becoming a case
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
Incidence rate - reporting
Measure of occurrence
Outcome
Population
Value
Incidence rate - limitations
Person-time not available
Complex to calculate
Which of the following would be the most appropriate numerator for determining the prevalence of measles on 31 December 2018?
The number of people with measles on 31 December 2018
A hypothetical study found the incidence of hospitalisation due to influenza in a population was 31.3 per 100,000 during 2017.
This is a measure of:
Incidence proportion
Brief recap
Prevalence:
existing cases – distribution, burden
Influenced by duration, does not tell us about development
Brief recap
Incidence proportion:
new cases - risk
Does not account for time-at-risk, increases if the time increases
Brief recap
Incidence rate:
new cases - speed
Needs person-time – not always available, complex
Prevalence, incidence and duration
Changes to incidence and duration can affect disease prevalence
Does the disease risk vary by age?
Yes, older people
The Gambia
29 per 100,000 for 2014
Do the age structures differ?
Population gambia younger ages
Population germany older ages
Age standardisation
Standardise both population to a standard population (remove effect of populations having different age structures)
Comparing populations
questions
Do the age structures differ?
Does the disease risk vary by age?
Age standardisation
The Gambia
29 per 100,000 for 2014
Germany
165 per 100,000 for 2014
86 per 100,000 for 2014
63 per 100,000 for 2014
when do you use Age standardisation?
Age structures differ
AND
Disease risk varies by age
