disease occurence lecture dr bony
TRANSCRIPT
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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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Menghitung frekuensi penyakit
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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Menghitung frekuensi penyakit
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