UKURAN ASOSIASI DALAM EPIDEMIOLOGI Nurul Wandasari Singgih Prodi Kesehatan Masyarakat Univ Esa Unggul 2012/2013

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? E D 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 Total a+b c+d Yes Exposure No Total a+c b+d a+b+c+d

Hypothetical Two-By-Two Table Lung cancer Yes No Total 370 715 70 300 15 700 Yes Smoking No Total 85 1,000 1,085

RR = incidence in the exposed = a/(a+b) 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 70 300 370 Smoking 15 700 715 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) Rasio Insidens Kumulatif (RIK)

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 Does Not Develop Exposed a b a + b Not c d c + d Incidence in exposed a a+b FIRST, = SELECT c c+d Incidence in non-exposed = a a+b c c+d Relative Risk ( RR ) = incidence in exposed incidence in non-exposed =

THEN FOLLOW UP TO SEE HOW MANY Example : A Prospective Study of 3,000 Smokers and 5,000 Non-smokers to Investigate Smoking and Coronary Heart Disease (CHD) THEN FOLLOW UP TO SEE HOW MANY Develop CHD Do Not Develop 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 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 merokok 70 395.594 17,7 Mantan perokok 65 232.712 27,9 Perokok 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 terjadi 1 – P = Probabilitas suatu kejadian tidak terjadi

Figure 11-5 A, Odds ratio (OR) in a cohort study 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) 400 700 1300 2000

Odds Ratio & Risk Ratio The odds ratio will provide a good estimate of the risk ratio when: The outcome (disease) is rare a / (a +b ) RR = ------------ c / (c +d) D+ D- E+ a b E- c d If the disease is rare, then cells (a) and (c) will be small OR = (a / c) / (b / d) a / (a +b ) a / b ad RR = ------------ = ------ =-- = OR c / (c +d) c / d bc OR = (ad) / (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.

Interpretation of Odds Ratio Relative odds associated with exposure OR = 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 Gerstman Chapter 8 (partial)