Lecture 16 - Measuring Disease Occurence Flashcards by kayla w

Why measure disease occurrence in

populations?

Health status
Trends over time
Impact among different groups

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Measures of occurrence

Prevalence

Incidence

incidence proportion (cumulative incidence)
incidence rate

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What is prevalence

proportion of a population who have the disease at a point in time

Point prevalence

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Prevalence why

Burden of disease

Resource allocation

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Prevalence calculation

Number of people with disease at given time / total number of people in that population in given time

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Prevalence reporting

Measure of occurrence - prevalence
Exposure of outcome - of disability 
Population - in people in NZ
Time point - in 2013
Value - 23.8%

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Prevalence - limitations

Difficult to assess the development of disease

2. Is influenced by the duration of the disease

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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

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Incidence proportion

proportion of an outcome-free population that develops the outcome of interest in a specified time period

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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

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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

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Incidence proportion

New cases during the time period / population at risk at the beginning of the time period

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Incidence proportion reporting

Measure of occurrence - incidence proportion 
Outcome - of low back pain
Population- in nurses
Time period - in 12 months
Value - was 35%

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Incidence proportion why

Risk (average)

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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)

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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?

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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

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Incidence rate reporting

Measure of occurrence - incidence rate
Outcome - glandular fever
Population - in class
Value - 50 per 100 person-years

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Incidence rate limitations

Person-time not available

Complex to calculate

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Brief recap what is prevalence

Existing cases - distribution, burden

Influenced by duration, does not tell us about development

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Brief recaps

What is incidence proportion?

New cases - risk

Doesn’t account for time-at-risk
Increases if time increases

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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