Post on 23-Apr-2017
UKURAN ASOSIASI
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
Arithmetic
• Suppose you have $2 and I have $1• Absolute comparisons made by
subtraction• $2 – $1 = $1• “I have $1 more than you”
• Relative comparisons made by division• $2 ÷ $1 = 2 [$ units cancel out]• “I have twice as much as you”
Arithmetic • Suppose that the 5-year rate of a
disease • in smokers is 2 per 100• in non-smokers is 1 per 100
• Absolute comparison: (2 per 100) – (1 per 100) = 1 per 100• There is one addition case per 100 smokers
• Relative comparison: (2 per 100) ÷ (1 per 100) = 2 [no units; “per 100” units cancel out]• Smokers are at twice the risk of non-
smokers
UKURAN ASOSIASI
• Relative : –RR (relative risk)
•Risk ratio•Rate ratio
–OR• Absolute :
–RD (risk difference)
Measures of Association
• How much greater the frequency of disease is in one group compared with another.
• Often presented in the form of a
two-by-two table.
Two-By-Two Table
a b
c d
DiseaseYes 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 cancerYes 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 StudyLung 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)
terpajantidakkelompokpadaRisikoterpajankelompokpadaRisikoRR
– Rasio Insidens Kumulatif (RIK)
terpajantidakkelompokpadakumulatif InsidensterpajankelompokpadakumulatifInsidensRIK
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
NotExposed c d c + d
FIRST,
SELECT
aa+b
cc+d
Incidence in exposed
Incidence in non-exposed
=
=
aa+b cc+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 :
DevelopCHD
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 5,00
0 17.4
THEN FOLLOW UP TO SEE HOW MANY
FIRST,SELECT
Ukuran-ukuran asosiasi• Ukuran rasio
– Rasio rate atau rasio densitas insidens (RDI)
terpajantidakkelompokpadainsidensDensitasterpajankelompokpadainsidens DensitasRDI
– Rasio Prevalens (RP)
terpajantidakkelompokpada PrevalensterpajankelompokpadaPrevalensRP
Perhitungan RR untuk IR
Outcome (+) Person-time
E (exposed) a N1
NE (unexposed) c N0
Total a + c T
IR pada populasi exposed (E) --- a/N1
RR=IR pada populasi unexposed (NE) --- c/N0
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 merokok 70 395.594 17,7Mantan 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.
Ukuran-ukuran asosiasi• Dari Tabel 1.• Hitunglah:
– Rasio rate atau rasio densitas insidens (RDI)
terpajantidakkelompokpadainsidensDensitasterpajankelompokpadainsidens DensitasRDI
8,217,749,6RDI
Postmenopausal Hormone Supplement and CHD
CHD Person-years
Ever use 30 54,308.7 Past use 19 24,386.7 Current 11 29,922.0Never 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
• Bisa diinterpretasikan sama seperti risiko relatif, dg syarat tertentu (misal: incidence rendah rare disease assumption)
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
P1PperistiwasuatuOdds
THEN, FOLLOW TO SEE WHETHER
Totals
Incidence rates
of Disease
Disease Develops
Disease Does Not Develop
Exposed a b a + b
NotExposed c d c + d
FIRST,SELECT
aa+b
cc+d
Incidence in exposed
Incidence in non-exposed
=
=
PROSPECTIVE STUDY
CASE – CONTROL (REPROSPECTIVE STUDY)
CASES
( with diseas
e)
CONTROLS ( without disease)
WereExposed a b
Were Not
Exposedc d
Totals a + c b + d
Proportion
exposed
aa+c
bb+d
FIRST,SELECT
THEN,MEASURE
PAST EXPOSUR
E
=
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
In case-control studies, the odds ratio is the ratio of the odds of the cases having been exposed to the odds of the controls having been exposed, i.e:
In prospective studies, the odds ratio is the ratio of the odds of the exposed people developing the disease to the odds non-exposed people developing the disease, i.e: a
b cd
=adbc
ab
cd =
adbc :
ac bd
=adbc
ac =
adbc : b
d
The odds ratio (Relative Odds)
Tabel 1. Tabulasi silang pemajan dan status sakit, insidens sakit dan Probabilitas odds sakit
pada studi kohort.Status sakit
Pemajan Sakit Tidak sakit
Total Insiden sakit (Risk)
Probabilitas odds sakit
+ a b a + b a/(a+b)
- c d c + d c/(c+d)
Total a + c b + d a+b+c+d
ba
baabaa
1
dc
dccdcc
1
Odds Ratio (OR) = Relative Odds = Cross Product Ratio pada studi kasus kontrol
Faktor Kasus Kontrol Total
Perokok 650 (a) 950 (b) 1600
Bukan perokok
50 (c) 350 (d) 400
Total 700 1300 2000
kontroluntukpemajanOddskasusuntukpemajanOddsRatioOdds
Odds Ratio (OR) = Relative Odds
cxbdxa
dbca
RatioOdds
kontroluntukpemajanOddskasusuntukpemajanOddsRatioOdds
8,450950350650
xx
cxbdxaRatioOdds
Perokok mempunyai risiko menjadi kasus 4,8 kali dari yang bukan perokok.Interpretasinya: odds perokok menjadi kasus 4,8 kali lebih besar dari odds bukan perokok
Prevalence Odds Ratio (POR) = Cross Product Ratio bila data didasarkan pada kasus-kasus
prevalens
Faktor Kasus Kontrol TotalPerokok 650 (a) 950 (b) 1600
Bukan perokok
50 (c) 350 (d) 400
Total 700 1300 2000
8,450950350650Prevalence xxRatioOdds
25,3125,0
40625,0400/501600/650)(Prevalence Ratioproportion
The odds ratio ( relative odds) is a good approximation of the relative risk when :
1.The cases are representative of all cases with regard to exposure;
2.The controls are representative of all control with regard to exposure;
3.The disease being studied is rare
=
Odds Ratio & Risk RatioThe odds ratio will provide a good estimate of therisk ratio when:
1. The outcome (disease) is rareD+ D-
E+ a bE- 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 directlyIn 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.
Incidence Odds Ratio (POR) = Cross Product Ratio bila data didasarkan pada kasus-kasus
insidens
Faktor Sakit Tidak sakit TotalPerokok 20 (a) 980 (b) 1000
Bukan perokok
10 (c) 990 (d) 1000
Total 30 1970 2000
02,29801099020I xxRatioOddsncidence
00,201,002,0
1000/101000/20)(Incidence Ratioriskproportion
Gerstman Chapter 8 (partial) 46
Interpretation of Odds Ratio• Relative odds associated with exposure
OR = 1 no associationOR > 1 positive associationOR < 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
RR < 1 RR = 1 RR > 1
Risk comparison between exposed and unexposed
Risk for disease is lower in the
exposed than in the unexposed
Risk of disease are equal for exposed and unexposed
Risk for disease is higher in the exposed than in the unexposed
Exposure as a risk factor for the disease?
Exposure reduces disease
risk(Protective
factor)
Particular exposure is not a
risk factor
Exposure increases
disease risk(Risk factor)
R ela tive riskR R O R
A ttrib u tab le ris kA R P A R
M easu re o fA ssoc ia tion
Gerstman Chapter 8 (partial) 49
Epidemiology Kept Simple
Chapter 8 Measures of Association
If it’s not clear…• Gordis Leon, M.D., M.P.H., Dr.P.H. 2009.
Epidemiolgy. W.B. Saunders Company. Philadelphia ,Chapter 11
• Epidemiology in Medicine. Henneckens CH, Buring JE. Edited by Mayrent SL, Chapter 4