DISEASE FREQ 2.ppt

8/13/2019 DISEASE FREQ 2.ppt

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INCIDENCE AND

PREVALENCEShirley I. M.



Measures of

Disease Frequency

The most basic measure of disease frequency is a simplecount of affected individuals.

However, counting is not enough!



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



So, in epidemiology we must know:

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

• the time period the information was

collected.



Basic Measure of Disease Frequency

Rate

an example: Incidence



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



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.



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.



Incidence

a type of rate

the number of new cases that develop in agroup of individuals during a specific timeperiod



Incidence (cont.)

I = number of new events during a period of time

number of persons at risk during this time period



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



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.



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



SMRs (Continued)

Calculation:

SMR = observed deaths

expected deaths



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



Example: Study of Smoking

and Lung Cancer (cont.)

Our study group had a risk of cancermortality approximately 35% greaterthan those in the general population.



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



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



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



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



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



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



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

DISEASE FREQ 2.ppt

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