Disease Occurence Lecture Dr Bony

7/25/2019 Disease Occurence Lecture Dr Bony

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Ukuran Penyakit

(Measures of Disease Occurence)

Bony Wiem Lestari

Departemen Epidemiologi & Biostatistik

2015


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Pemilihan ukuran penyakit

1. Ditentukan oleh desain studi (cross-sectional,

case-control, cohort)

2. Ditentukan oleh pertanyaan penelitian

(research questions)


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Menghitung frekuensi penyakit

So, in epidemiology we must know:

the size of the population from which theaffected individuals come, and

the time period the information was collected.

Why is a simple count not enough?

Three cases of cancer per year from a city of 1,000 people isvery different than 3 cases per year from a city of 100,000

people


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Closed Cohort Study

Cumulative

incidence in

exposed =

X1/N1

Cumulative

incidence in

unexposed =

X0/N0

Type of population: fixed (membership is permanent)


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Dynamic Cohort Study

Incidence

rate inexposed =

X1/P1

Incidence

rate in

unexposed =X0/P0

Type of population: dynamic (membership is transitory)


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Types of Calculations

Types of

calculations

Characteristics

Ratio Division of two unrelated numbers

Proportion Division of two related numbers;

numerator is a subset of

denominator

Rate Division of two numbers; time is

always in denominator


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Rate = Number of events in a specified time period

Average population during the time period

***the measure of time is a critical part of a rate!

Such as, the number of newly diagnosed cases of

breast cancer per 100,000 women during 1999.


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Incidence Rate

The number of new events (cases) divided by theamount of person time in the base population. It can bemeasured in closed or open populations. It is often

restricted to include a maximum of one event perperson.

= number of new events in a specified period

number of persons exposed to risk during this period Units = must always include a unit of time, example:

cases per 100.000 person years


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Incidence Proportion

or Cumulative Incidence

The average risk for a population

The probability that individuals in the population get thedisease during the specified period

It is measured only in closed population

= number of people who get a disease during a specified periodnumber of people free of the disease in the population at risk at

the beginning of the period


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Prevalence

Point prevalence

The proportion of a population that has disease atgiven a specified point on the time scale.

Period Prevalence/ Interval prevalence

The total number of persons known to have had the

disease or attribute at any time during specifiedperiod.

Annual Prevalence


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Perbedaan Incidence & Prevalence

Measure Type of

number

Units Range Type of cases Major uses

Cumulative

Incidence

Proportion None 0 to 1 New Research on

causes,

incidence,

prevention,and treatment

of disease

Incidence

rate

True rate 1/time 0 to infinity New Research on

causes,

prevention,

and treatment

of disease

Prevalence Proportion None 0 to 1 Existing Resource

planning


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Exercise

Point

Prevalence

at T1 ? Cumulative

incidence

from T0-

T1? Incidence

rate from

T0-T1?


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Answer:

1. Point Prevalence at T1 ?

= = 0.5

2. Cumulative incidence from T0-T1?= 3/5 = 0.6

3. Incidence rate from T0-T1?

= 3/(5+1+4+3+1)=3/14 = 0.21 cases per year


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THE INCIDENCE CHARACTERISTICS

1. Indicated for the new events or cases occurred inthe population

2. A fundamental tool in etiological study of disease

3. Act as direct indicator of the risk of getting sick(the probability of getting sick)

4. The change in incidence rate means the change ofetiological balance

5. The Incidence Rate obtained accurately by cohortstudy


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THE CHARACTERISTICS OF PREVALENCE

1. The subjects are all cases / events in population

2. Depend on the incidence and duration of thedisease

3. The low Prevalence Rate because of low incidence,short duration of disease or fast cured of thedisease

4. Therapy to prevent the death but can not omit thedisease might increasing the prevalence of thedisease


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THE CHARACTERISTICS OF PREVALENCE

5. In chronic disease the prevalence = I x D

6. It is useful to describe the chronic disease

7. It is an important and useful measure of the burdenof disease in a community

8. Determined by a single survey (cross sectionalstudy)

9. It is valuable for planning health services.


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Prevalence, Incidence and Mean

duration of illness

Disease Occurence Lecture Dr Bony

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