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Public Health > Measuring Disease Frequency > Flashcards

Measuring Disease Frequency Flashcards

1
Q

Importance of disease frequency?

A

-Can start understanding size of problem
-Can spot disease outbreaks
-Assess risks & management of conditions
-Aids resource planning
->Are areas of healthcare failing? ->Do they require more focus?

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2
Q

How should a case (i.e., 1 person with a disease) be defined, & why in this way?

A

-Standardised classification
-Exact definition = dependent on data source & use

->must be justifiable method
->must be measurable
->must be reproducible
-must be comparable
=> so can compare different places & so conclusions of others

Think of COVID - what did 1 case mean?

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3
Q

How is the population at risk defined?

A

-People who would be a case if developed the disease
-Person time @ risk

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4
Q

What were the 3 case types?

A

-Possible case
-Probable case
-Confirmed case

—> but all should be explained & justified

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5
Q

What is a problem which will arise with all measures of cases?

A

-Missing cases
-Including non-cases

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6
Q

4 requirements for case definitions?

A

-Reproducible
-Clear
-Specific
-Measurable

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7
Q

What are the 2 types of population definitions?

A

-Fixed
-Dynamic

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8
Q

What is a fixed population?

A

= has specific criteria (can’t change)

> Membership is permanent
-Attended a specific event
-Born @ certain time

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9
Q

What is a dynamic population?

A

= more flexible (can change)

> Membership is based current status
-Currently studying at uni
-Lives in specific city

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10
Q

Basic mathematical methods to measure frequency?

A

*Count
*Ratio
*Proportion
*Rate

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11
Q

What are the ways to measure frequency?

A

-Prevalence
-Incidence

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12
Q

What is prevalence (measure of frequency)?

A

-Proportion
-Not a rate – no time component in calculation
-Measures proportion of existing disease in pop @ given time
-Snapshot -> no time component
-Dimensionless, +ve no. (0 to 1)

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13
Q

What are the 2 types of disease prevalence?

A

-Point
-Period

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14
Q

What is point prevalence?

A

Proportion of a pop who have a disease @ any 1 time

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15
Q

Calculation for disease point prevalence?

A

No. cases @ specific date/No. cases @ same date

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16
Q

8/11/2022: 600 people have condition from
population of 1300 - calculate point prevalence?

A

600/1300 = 0.046 x100 = 4.6%

= 0.46 people for every 10
= 4.6 people for every 100*** (same as % - per 100)
= 46 people for every 1000
= 460 people for every 10000

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17
Q

What is period prevalence?

A

Proportion of a population with a disease during a specified time period

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18
Q

Calculation for period prevalence?

A
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19
Q

How does point, period (1 month), period (1 year), period (lifetime) fit together?

A
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20
Q

When is period prevalence more useful & why?

A

When is over a longer period of time - as will capture more people

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21
Q

Why is prevalence estimation useful?

A

-Determination of sickness load
-Planning health services
-Examining health behs

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22
Q

What factors affect disease prevalence?

A

-Disease duration
-Case fatality
-Changes in treatment
-Incidence
-Ascertainment of cases
-Migration

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23
Q

How does disease duration affect disease prevalence?

A

*Short = may miss some people - snapshot
*Long = better - should capture more people

24
Q

How does disease/case fatality affect disease prevalence?

A

-If disease is more fatal = more deaths so prevalence decreases but incidence will stay same
(some people will be cured)
-If disease is less fatal = fewer deaths so prevalence increases - especially if is chronic condition so can’t cure

—> all depends on if can cure illness & if people are likely to die from it (2 ways out of prevalence)

25
Q

What are the 2 ways out of prevalence?

A

-Cured
-Death

26
Q

Define incidence of a disease.

A

Number of new cases

27
Q

Define prevalence of a disease.

A

Number of existing cases

28
Q

What is incidence?

A

-Measures the occurrence of new cases in a population at risk over time
-Can be measured as a proportion or a rate
-Most fundamental epidemiologic indicator
->Measures force of morbidity (as a rate)
->Measures conversion of health status (proportion /rate)

29
Q

How is incidence measured?

A

-Incidence proportion - cumulative incidence
-Incidence rate
-Person time

30
Q

What is incidence proportion?

A

-Synonyms = incidence, cumulative incidence, risk
-Measures probability (risk) of developing disease during
period of observation
-Dimensionless, +ve no. (0 to 1)

31
Q

What is incidence proportion - cumulative incidence?

A

Proportion of people who develop a disease during a specific period of time

32
Q

Calculation for incidence proportion - cumulative incidence?

A

***Remove people who alarmed at have the disease - as are no longer @ risk in denominator)

No. new cases over period/No. pop at risk over same period

33
Q

Why is incidence proportion - cumulative incidence useful?

A

-Appropriate for fixed pops & short follow-up (not dynamic)
-Must specify time period of observation as risk
changes w/ time
-Not appropriate for long-term follow-up due to potential loss of subjects
-Assuming: complete follow-up, same risk over time

—> some diseases show more stable risk rates (CVD vs flu)

34
Q

Total number of new cases
-Between 2016 – 2020: 4572 new cases out of at-risk population of 83270 - calculate cumulative incidence?

A

4572/83270 = 0.055 x 100 = 5.5%

(If had been given total pop not just pop @ risk - would have had to subtract no. new cases)

= 0.55 people for every 10
= 5.5 people for every 100 (same as % as us per 100)
= 55 people for every 1000
= 550 people for every 10000
-Total no. new cases for period = 5.5%

35
Q

Why is estimation of incidence proportion useful?

A

-Surveillance of a disease
-Understanding aetiology
-Planning service use

36
Q

What is incidence rate?

A

-Measures the speed that new cases develop during specified time period
-Cases per person-time = what measure
-Synonyms = incidence, incidence density, rate
-Follow-up may be incomplete
-Risk period not same for all subjects

37
Q

Calculation for incidence rate?

A
38
Q

What is person-time (denominator in incidence rate)?

A

= all the time each person is in the pop at risk
-Time for each person = person-time
-Sum of person-times = total person-time at risk for pop

39
Q

What does this chart mean?

A

5 people in pop followed up for 5 years - to find out incidence rate
-Each shaded square = 1 year contributed to study
—> contribution to study means not having a stroke - so 2/5 people didn’t suffer a stroke at all in the 5 years
-If due = no longer contributing

1+5+5+3+2 = 16
So total length of time each person in pop is at risk =16 person-years

40
Q

Calculate incidence rate when - follow 1000 patients aged 70 and over registered in 1 GP practice at monthly intervals to measure development of respiratory infection in one year

-50 adults developed respiratory infection -950 complete the year disease free
-25 complete 3 months (0.25 years) before infection
-25 complete 6 months (0.5 years) before infection

A

50/(950)+(0.25x25)+(0.5x25) = 50/968.75 = 0.052 x 100 = 5.2

5.2 respiratory infections per 100 years (as is %)

41
Q

23 cases of colorectal cancer were diagnosed in 35,532 people followed-up for 105,586 person-years. What is the incidence rate?

A

23/(105586) = 0.00021783 x10000 = 2.1783

= 2.2 per 10,000 person-years

42
Q

What are cross-sectional (prevalence) surveys?

A

-Observational & descriptive
-Survey - gives snapshot of health in
(sample of) well-defined pop
-Uses point or prevalence estimate to analyse

43
Q

What are the key elements of cross-sectional (prevalence) surveys?

A

-Identification of the target population
-Procedure for sampling
-Method of data collection
-Definitions used

44
Q

+ves of cross-sectional (prevalence) surveys?

A
45
Q

-ves of cross-sectional (prevalence) surveys?

A
46
Q

What are cohort studies?

A

-Observational & analytical
-Identify group(s) of individuals & measure exposure status; follow forward in time to ascertain occurrence of disease(s) of interest
-Starts w/ exposures & looks forward to possible effects

—> follow whole cohort & see what cases develop

47
Q

+ves of cohort studies?

A
48
Q

-ves of cohort studies?

A
49
Q

What is the period effect?

A

Something in a set time frame that affects pop
E.g., COVID

50
Q

What is the cohort effect?

A

Whole cohort affected by a particular event

51
Q

What factors affect prevalence?

A

-Time of measurement
-Change case definition
-Introduce/effect of intervention or treatment
-Changes in duration or incidence of a disease
-Dynamic pop

52
Q

What factors affect incidence?

A

-New risk factor
-Changes in virulence
-Migration
-Change in intervention strategy

53
Q

What are the elements of uncertainty that can affect variations shown?

A

-Illusion not real epidemiological association. E.g.
•Chance
•Error
•Data collection / coding
•Analysis

54
Q

Compare incidence VS prevalence

A
55
Q

What are some key indicators of health?

A

-Mortality indices:
*crude mortality rate *Age-specific mortality rates
*Infant mortality rate *Child mortality rate

-Life expectancy
-Healthy life expectancy/Disability adjusted life years -Morbidity rates
-Impact-quality of life, wellbeing, work

56
Q

Calculate the prevalence of angine cases at baseline for men & women.

A

Men = 164/6895 x 100 = 2.38%

Women = 134/2413 x 100 = 3.93%

57
Q

Calculate the cumulative incidence of angina during the study follow-up period for men and women

A

Men = 177/(6895-164) = 177/6731 x 100 = 2.63%

Women = 151/(3413-134) = 151/3279 x 100 = 4.61%

Public Health (10 decks)

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