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Pop health > Lecture 16 - Measuring Disease Occurence > Flashcards

Lecture 16 - Measuring Disease Occurence Flashcards

1
Q

Why measure disease occurrence in

populations?

A

Health status
Trends over time
Impact among different groups

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2
Q

Measures of occurrence

A

Prevalence

Incidence

  • incidence proportion (cumulative incidence)
  • incidence rate
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3
Q

What is prevalence

A

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

Point prevalence

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4
Q

Prevalence why

A

Burden of disease

Resource allocation

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5
Q

Prevalence calculation

A

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

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6
Q

Prevalence reporting

A 
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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7
Q

Prevalence - limitations

A
  1. Difficult to assess the development of disease

2. Is influenced by the duration of the disease

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8
Q

Incidence

A

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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9
Q

Incidence proportion

A

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

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10
Q

Incidence proportion - calculation

A

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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11
Q

Why might people not be considered ‘at risk’ at the start of a study?

A
  • They already have the condition

* The condition is something that they cannot develop

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12
Q

Incidence proportion

A

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

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13
Q

Incidence proportion reporting

A 
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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14
Q

Incidence proportion why

A

Risk (average)

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15
Q

Incidence proportion limitations

A

• 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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16
Q

What is Incidence rate

A

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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17
Q

Incidence rate calculation

A

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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18
Q

Incidence rate reporting

A

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

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19
Q

Incidence rate limitations

A

Person-time not available

Complex to calculate

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20
Q

Brief recap what is prevalence

A

Existing cases - distribution, burden

Influenced by duration, does not tell us about development

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21
Q

Brief recaps

What is incidence proportion?

A

New cases - risk

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

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22
Q

Brief recap

What is incidence rate?

A

New cases - speed

Needs person-time - not always available, complex

23
Q

Prevalence of gout in NZ

A

Shows burden of disease among different groups of a population

Higher in males, pacific, maori than european and females

24
Q

Prevalence aspects

A 
Disease
Health condition
Exposure 
- Possible determinant of disease 
- Eg smoking
25
Q

Prevalence - reporting

A 
Measure of occurrence
Exposure or outcome
Population
Time point
Value
26
Q

Measure of occurrence

A

incidence rate
incidence proportion
prevalence

27
Q

Exposure or outcome

A

health condition

28
Q

Population

A

people in nz

29
Q

what is incidence proportion?

A

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

30
Q

Why incidence repost

A

Risk
Shows who have the highest risk
Average risk in that group

31
Q

Incidence proportion - reporting

A 
Measure of occurrence
Outcome
Population
Time period
Value
32
Q

Treatment might mean people recover faster

A

Disease shorter duration

Low prevalence

33
Q

Treatment might mean people may live longer with the disease

A

Increase duration

Increase prevalence

34
Q

Incidence proportion - limitations

A
  • Assumes a ‘closed’ population (does not account for people coming or going)
  • Highly dependent on the time period (longer time period = higher incidence proportion)
35
Q

what is incidence rate?

A

rate at which new cases of the outcome of interest occur in a population

36
Q

why use incidence rate?

A

How quickly are new cases of the condition developing in the population?

37
Q

Incidence rate - calculation

A

Number of people who develop the disease in a specified period / Number of person-years at risk of developing the disease

38
Q

person-years

A

Sum of everyone in the population’s time at risk of becoming a case

39
Q

Why might someone stop being ‘at risk’?

A
  • They become a case
  • They are lost to follow-up (e.g. die, move away, no longer take part)
  • Follow-up time ends
40
Q

Incidence rate - reporting

A

Measure of occurrence

Outcome

Population

Value

41
Q

Incidence rate - limitations

A

Person-time not available

Complex to calculate

42
Q

Which of the following would be the most appropriate numerator for determining the prevalence of measles on 31 December 2018?

A

The number of people with measles on 31 December 2018

43
Q

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:

A

Incidence proportion

44
Q

Brief recap

Prevalence:

A

existing cases – distribution, burden

Influenced by duration, does not tell us about development

45
Q

Brief recap

Incidence proportion:

A

new cases - risk

Does not account for time-at-risk, increases if the time increases

46
Q

Brief recap

Incidence rate:

A

new cases - speed

Needs person-time – not always available, complex

47
Q

Prevalence, incidence and duration

A

Changes to incidence and duration can affect disease prevalence

48
Q

Does the disease risk vary by age?

A

Yes, older people

49
Q

The Gambia
29 per 100,000 for 2014
Do the age structures differ?

A

Population gambia younger ages

Population germany older ages

50
Q

Age standardisation

A

Standardise both population to a standard population (remove effect of populations having different age structures)

51
Q

Comparing populations

questions

A

Do the age structures differ?

Does the disease risk vary by age?

52
Q

Age standardisation

The Gambia
29 per 100,000 for 2014

Germany
165 per 100,000 for 2014

A

86 per 100,000 for 2014

63 per 100,000 for 2014

53
Q

when do you use Age standardisation?

A

Age structures differ
AND
Disease risk varies by age

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