UKURAN ASOSIASI DALAM EPIDEMIOLOGI

Putri Handayani, M. KKK

Tipe ukuran yang digunakan dalam epidemiologi

• Ukuran asosiasi– Merefleksikan kekuatan atau besar asosiasi

antara suatu eksposur/faktor risiko dan kejadian suatu penyakit

– Memasukkan suatu perbandingan frekuensi penyakit antara dua atau lebih kelompok dengan berbagai derajat eksposur

– Beberapa ukuran assosiasi digunakan untuk mengestimasi efek

Basic Question in Analytic Epidemiology

• Are exposure and disease linked?

Exposure Disease

Ukuran-ukuran asosiasi

• Ukuran rasio (perbandingan relatif)

– rasio dua frekuensi penyakit membandingkan kelompok terpajan dengan kelompok tidak terpajan

• Ukuran perbedaan efek (perbandingan absolut)

– perbedaan antara ukuran frekuensi penyakit suatu kelompok terpajan dan kelompok yang tidak terpajan

Two-By-Two Table

a b

c d

Disease

Yes No

Yes

Exposure

No Total a+c b+d

Total

a+b

c+d

a+b+c+d

Hypothetical Two-By-Two Table

70 300

15 700

Lung cancer

Yes No

Yes

Smoking

No

Total 85 1,000

Total

370

715

1,085

Relative Risk (RR)

• Measures how likely the exposed group will develop a disease compared to the unexposed group.

RR = incidence in the exposed = a/(a+b)

incidence in the unexposed c/(c+d)

Example: Hypothetical Study

Lung cancer

Yes No Total

Yes 70 300 370

Smoking No 15 700 715

Total 85 1,000 1,085

Relative Risk = 70/(70+300) = 9.0

15/(15+700)

Which means… participants who smoked were 9 times more likely to develop lung cancer than those who did not smoke.

Interpreting Measures of Association

RR of 1.0 indicates that the occurrence of disease in the exposed an unexposed groups are identical:

–No association observed between exposed and unexposed groups.

Interpreting Measures of Association (Continued)

• RR greater than 1.0 indicates a positive association, or an increased risk among the exposed.

• RR less than 1.0 means that there is a decreased risk among the exposed group.

Ukuran-ukuran asosiasi

• Ukuran rasio

– Rasio risiko atau risiko relatif (RR)

terpajantidakkelompokpadaRisiko

terpajankelompokpadaRisikoRR

– Rasio Insidens Kumulatif (RIK)

terpajantidakkelompokpadakumulatif Insidens

terpajankelompokpadakumulatifInsidensRIK

Perhitungan RR untuk CI

Outcome (+) Outcome (-) Total

E (exposed) a b a + b

NE (unexposed) c d c + d

Total a + c b + d N

(a + b + c + d)

CI pada populasi exposed (E) --- a/(a + b)

RR=

CI pada populasi unexposed (NE) --- c/(c + d)

THEN, FOLLOW TO SEE WHETHER

Totals

Incidence

rates

of

Disease

Disease

Develops

Disease

Does Not

Develop

Exposed a b a + b

Not

Exposedc d c + d

FIRST,SELECT

a

a+b

c

c+d

Incidence in

exposed

Incidence in

non-exposed=

=

a

a+b

c

c+d

Relative Risk ( RR ) = incidence in exposed incidence in non-exposed =

A Prospective Study of 3,000 Smokers and 5,000 Non-

smokers to Investigate Smoking and Coronary Heart Disease

(CHD)

Example :

Develop

CHD

Do Not

Develop

CHD

Total

Incidence

per 1,000

per year

Healthy Smokers 84 2,916 3,000 28.0

Healthy Non-Smokers 87 4,913 5,000 17.4

THEN FOLLOW UP TO

SEE HOW MANY

FIRST,SELECT

Contoh 5.

Tabel 1. Kaitan antara merokok dan angka insidens stroke dalam suatu kohort.

Kategori

merokok

Jumlah kasus

stroke

Orang-tahun

observasi

(lebih dari 8

tahun)

Tingkat

insidens stroke

(per 100.000

orang tahun)

Tidak pernah

merokok70 395.594 17,7

Mantan perokok 65 232.712 27,9Perokok 139 280.141 49,6

Total 274 908.447 30,2

Sumber: diterjemahkan dari:Beaglehole et al. Basic Epidemiology. WHO. 1993. 18.

Postmenopausal Hormone Supplement and CHD

CHD Person-years

Ever use 30 54,308.7

Past use 19 24,386.7

Current 11 29,922.0

Never use 60 51,477.5

RR ever use vs never use = (30/54,308.7) / ( 60/51477.5) = 0.5

RR past use vs never use = (19/24386.7) / (60/51477.5) = 0.7

RR current use vs never use = (11/29922.0) / (60/51477.5) = 0.3

Ukuran-ukuran asosiasi

• Ukuran rasio

– Rasio odds (Odds ratio = OR)

• Nama lain: Odds relative; rasio kros-produk

• rasio dua odds yang digunakan dalam studi kasus-kontrol untuk mengestimasi rasio rate atau rasio risiko

Ukuran-ukuran asosiasi

• Ukuran rasio

– Rasio odds (Odds ratio = OR)

• odds untuk satu kelompok dibagi dengan odds untuk kelompok yang lain

• Mempunyai interpretasi yang sama seperti risiko relatif

Ukuran-ukuran asosiasi

• Odds suatu kejadian

– rasio probabilitas bahwa kejadian terjadi terhadap probabilitas kejadian tidak terjadi

P = Probabilitas suatu kejadian terjadi1 – P = Probabilitas suatu kejadian tidak terjadi

P1

PperistiwasuatuOdds

Figure 11-5 A, Odds ratio (OR) in a cohort study. B, Odds ratio (OR) in a case-control study.

Downloaded from: StudentConsult (on 8 October 2009 11:44 AM)

© 2005 Elsevier

Prevalence Odds Ratio (POR) = Cross Product Ratio bila data didasarkan pada kasus-kasus prevalens

Faktor Kasus Kontrol Total

Perokok 650 (a) 950 (b) 1600

Bukan

perokok

50 (c) 350 (d) 400

Total 700 1300 2000

8,450950

350650Prevalence

x

xRatioOdds

25,3125,0

40625,0

400/50

1600/650)(Prevalence Ratioproportion

Odds Ratio & Risk RatioThe odds ratio will provide a good estimate of therisk ratio when:

1. The outcome (disease) is rare

D+ D-

E+ a b

E- c d

OR = (a / c) / (b / d)

OR = (ad) / (bc)

a / (a +b )RR = ------------

c / (c +d)

If the disease is rare, thencells (a) and (c) will be small

a / (a +b ) a / b adRR = ------------ = ------ =-- = OR

c / (c +d) c / d bc

Figure 11-6 Example: The odds ratio is a good estimate of the relative risk when a disease is infrequent.

Downloaded from: StudentConsult (on 8 October 2009 11:44 AM)

© 2005 Elsevier

Figure 11-7 Example: The odds ratio is not a good estimate of the relative risk when a disease is not

infrequent.Downloaded from: StudentConsult (on 8 October 2009 11:44 AM)

© 2005 Elsevier

In a prospective study/cohort study,

the Relative Risk can be calculated

directly

In a retrospective study /case control

study , the RR cannot be calculated

directly, so that the Relative Odds or

ODDS RATIO ( Cross Products Ratio )

is used as an estimate of the RR, when

the risk of the disease is low

OR & RR

• Pada penyakit yang jarang terjadi,nilai Odds Ratio hampir sama dengan nilai Relative Risk (Risk Ratio). Nilai Prevalence Odds Ratio hampir sama dengan nilai Prevalence Proportion Ratio.

• Pada penyakit yang umum terjadi, nilai Odds Ratio lebih ekstrim dari pada Risk Ratio.

Gerstman Chapter 8 (partial) 28

Interpretation of Odds Ratio

• Relative odds associated with exposureOR = 1 no association

OR > 1 positive association

OR < 1 negative association

• Size of OR indicates strength of association

• OR ≈ RR when disease rare (i.e., risk < 5%); when disease not rare, OR still a valid measure of association