DISEASE FREQ 2.ppt
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Transcript of DISEASE FREQ 2.ppt
8/13/2019 DISEASE FREQ 2.ppt
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INCIDENCE AND
PREVALENCEShirley I. M.
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Measures of
Disease Frequency
The most basic measure of disease frequency is a simplecount of affected individuals.
However, counting is not enough!
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Why is a simple count not enough?
3 cases of cancer per year from a city of1,000 people is very different than 3 casesper year from a city of 100,000 people
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So, in epidemiology we must know:
• the size of the population from which theaffected individuals come, and
• the time period the information was
collected.
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Basic Measure of Disease Frequency
Rate
an example: Incidence
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Rate
basic measure in epidemiology
the frequency with which an event occurs in agroup of people
used to compare the occurrence of disease indifferent groups
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Example: Rates
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 breastcancer per 100,000 women during 1999.
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Common Rates
mortality (death) rate is the number of deaths in adefined group of people during a specified timeperiod.
birth rate is the number of live births in a definedgroup of people over a specified time period.
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Incidence
a type of rate
the number of new cases that develop in agroup of individuals during a specific timeperiod
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Incidence (cont.)
I = number of new events during a period of time
number of persons at risk during this time period
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Example: Incidence
If there were 150,00 new cases of lung cancer in theUnited States during 1997, the incidence rate
would be:
(150,000/260,000,000) = 0.000058
0.000058 x 100,000 people
= 58 cases per 100,000 people, per year
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Measures of Association
How much greater the frequency of disease is in onegroup compared with another.
Often presented in the form of a two-by-two table.
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Standardized Mortality Ratios
(SMRs)
the ratio of the observed number of deaths to theexpected number of deaths
a standard group of people is used to determine theexpected number of deaths the standard is often the US population, a state, or a county.
serves as the comparison group
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SMRs (Continued)
Calculation:
SMR = observed deaths
expected deaths
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Example: Study of Smoking
and Lung Cancer
Our hypothetical study found 58 lung cancer deaths between 1948 and
1963:
based on US population rates, we know that 42.9 cancer deaths were expected in a similar population.
SMR = (58/42.9) =1.35
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Example: Study of Smoking
and Lung Cancer (cont.)
Our study group had a risk of cancermortality approximately 35% greaterthan those in the general population.
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Population at Risk
Adalah bagian dari populasi yang rentanterhadap suatu penyakit
Dapat ditentukan faktor demografi lingkungan
Sebagian besar pengukuran penyakit didasarkan
pada konsep prevalens dan insidens
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Prevalens dan Insidens
Prevalens suatu penyakit adalah jumlah kasus padasuatu populasi tertentu pada suatu waktu tertentu
Insiden adalah jumlah kasus baru pada suatu periode waktu tertentu pada suatuy populasi
Data prevalens dan insidens menjadi lebih bermanfaatbila dikonversi ke dalam bentuk rate.
Rate dihitung dengan membagi jumlah kasus denganjumlah population at risk dan diekspresikan sebagai
kasus per 10n
individu
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Prevalens rate
jumlah individu denga penyakit/keadaan padasuatu waktu tertentu/
jumlah individu pada population at risk pada suatu waktu tertentu
Bila data dikumpulkan pada suatu waktu tertentudisebut point prevalence
Bila selama suatu waktu tertentu disebut period
prevalence, denominatornya population at risk padapertengahan periode
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Faktor yang meningkatkan prevalens
Lamanya penyakit panjang
Meningkatkan kasus baru
In-migration kasus
Out migration dari individu yang sehat In-migration individu yg suseptibel
Fasilitas diagnostik lebih baik
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Insidens rate
Jumlah individu yang menderita oenyakit pada suatu waktu tertentu/
jumlah lamanya waktu selama setiap individu padapopulation at risk dalam risiko
Case fatality Adalah pengukuran beratnya penyakit
Case fatality %:
Jumlah kematian karena suatu penyakit/jumlah kasuspenyakitnyang sama pada periode yang sama
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Mortalitas
Crude death rate/crude mortality rate Jumlah kematian pada suatu waktu tertentu/ rata-rata total
populasi selama periode tersebut
Death rate dapat juga diekspresikan untuk kelompokyang lebih spesifik seperti umur, jenis kelamin, ras Jumlah kematian yg terjadi pada umur, jenis kel tertentu dari
populasi pada suatu wilayah dan waktu tertentu/
Jumlah total populasi pada kelompok umur, jenis kelamin yg
sama pada wilayah yg sama dan periode yang sama
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Infant mortality death
Jumlah kematian anak kurang dari 1 tahun/ Jumlah lahir hidup dalam tahun yang sama
Maternal mortality rate
Kematian maternal yang berhubungan dengankehamilan dalam 1 tahun/
Total kelahiran hidup pada tahun yang sama