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OBSERVERENDE UNDERSØGELSER

OBSERVERENDE UNDERSØGELSER. Kim Overvad Institut for Epidemiologi og Socialmedicin Aarhus Universitet Forår 2002. Epidemiologisk design. Observerende undersøgelser beskrivende: Undersøgelsesenheden er populationer regional variation migrationsundersøgelser korrelationsundersøgelser

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OBSERVERENDE UNDERSØGELSER

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  1. OBSERVERENDE UNDERSØGELSER Kim Overvad Institut for Epidemiologi og Socialmedicin Aarhus Universitet Forår 2002

  2. Epidemiologisk design Observerende undersøgelser • beskrivende: Undersøgelsesenheden er populationer • regional variation • migrationsundersøgelser • korrelationsundersøgelser • tidsrækker • analytiske: Undersøgelsesenheden er individer • tværsnitsundersøgelser • follow-up undersøgelser • case-kontrol undersøgelser Eksperimentelle undersøgelser

  3. Korrelationsundersøgelser Biologi • rationale • tid Ikke nødvendigvis personsammenfald • niveau fejlslutning Confounding • variation i andre risikoindikatorer

  4. Populationserfaring • Sampling • tværsnitsundersøgelse • N • follow-up undersøgelse • eksponerede og ikke-eksponerede (n+ og n-) • case-kontrol undersøgelse • oftest syge og raske (a + c og b + d)

  5. Cross – Sectional Study, 1 A cross-sectional study was carried out among women attending a university health service to investigate the determinants of cervical human papillomavirus (HPV) infection. A sample of 467 women were asked to complete a self-administered questionaire on socio-demographic variables and sexual behaviour at the time of their visit to the clinic. The prevalence of HPV infection was then examined in relation to marital status and lifetime number of male sexual partners (Ley et al., 1991).

  6. Cross – Sectional Study, 2

  7. Cross – Sectional Study, 3 The prevalence proportion ratio for each exposure level was calculated by forming 2 x 2 tables as illustrated below for women with 10+ partners Prevalence proportion among women with 10+ partners = 70/102 = 68.6% Prevalence proportion among women with one partner = 19/90 = 21.1% Prevalence proportion ratio = 68.6% / 21.1% = 3.3

  8. Follow-up Study, 1 The Nurses’ Health Study is a cohort study of 121,700 US Female registered nurses aged 30-55 years when the cohort was established in mid-1976. A total of 1,799 newly diagnosed breast cancer cases were identified during the first 10 years of follow-up from mid-1976 to mid-1986. Analyses were then conducted to investigate the relationship between oral contra- ceptive use and risk of breast cancer. (Romieu et al., 1989).

  9. Follow-up Study, 2 The incidence rates of breast cancer among nurses aged 45-49 years at the time of their entry into the cohort was examined in relation to use of oral contraceptives Data from Romieu et al. (1989) Rate ratio = 217 per 100,000 pyrs/187 per 100,000 pyrs = 1.16 95% confidence for the rate ratio = 0.96 to 1.40 Rate difference = 217 per 100,000 pyrs – 187 per 100,000 pyrs = 30 per 100,000 pyrs 95% confidence interval for the rate difference = -8 to 68 per 100,000 pyrs

  10. Fordele ved observerende follow-up undersøgelser Fordele • direkte identifikation af studiebasen • indflydelse på eksponeringsfordelingen • kvalitetskontrol af data • eksponeringsoplysninger før sygdom • flere sygdomsudfald • mulighed for beregning af flere associationsmål

  11. Ulemper ved observerende follow-up undersøgelser Ulemper • ressourcer • tid • penge • enkelt eksponeringer • påvirkning af den diagnostiske proces eller sygdomsregistreringen • etik

  12. Oplysninger om eksponering Hypoteser Muligheder • specifik (tid og sted) • gennemsnit • max/min • kumuleret

  13. Associationsmål Follow-up undersøgelse, komplet follow-up • Relativt associationsmål • relativ risiko • RR = a/n+ / c/n- • Absolut associationsmål • risiko differens • RD = a/n+ – c/n-

  14. Associationsmål Follow-up undersøgelse, inkomplet follow-up • Relativt associationsmål • incidens rate ratio • IRR = a/t+ / c/t- • Absolut associationsmål • incidensrate differens • IRD = a/t+ – c/t-

  15. Valg af associationsmål i follow-up undersøgelsen Sygdomshyppigheden blandt ikke-eksponerede, et relativt og et absolut associationsmål supplerer hinanden.

  16. Follow-up undersøgelse Ved sjældne sygdomme er n+ og n- samt b og d meget større end a og c

  17. Case-Control Study, 1 A population-based case-control study was carried out in Spain and Colombia to assess the relationship between cervical cancer and exposure to human papillomavirus (HPV), selected aspects of sexual and reproductive behaviour, use of oral contraceptives, screening practices, smoking, and possible interactions between them. The study included 436 incident cases of histologically confirmed invasive squamous-cell carcinoma of the cervix and 387 control of similar age randomly selected from the general Population that generated the cases (Muñoz et al., 1992a).

  18. Case-Control Study, 2 Risk of developing cervical cancer in relation to the lifetime number of sexual partners.

  19. Case-Control Study, 3 The odds ratios for each category of exposure were calculated in the following way: Odds ratio = (125/265) / (74 x 305) = 1.94

  20. Fordele ved case-kontrol undersøgelser Fordele • ressourcer • tid • penge • flere eksponeringer • hypotese genererende • etik

  21. Ulemper ved case-kontrol undersøgelser Ulemper • afgrænsning af studiebasen • kvalitetskontrol af data • eksponering • effekt • enkelt sygdomme • ikke økonomisk ved studiet af sjældne eksponeringer • associationsmål

  22. Studiebasen Studiebasen er den populationserfaring, vi ønsker at beskrive på baggrund af undersøgelsen Studiebasen er de personer, for hvem det gælder, at de ville blive cases, givet de udviklede den sygdom, der studeres

  23. Eksponering Eksponeringssandsynlighed Muligheder for at øge eksponeringssandsynligheden • restriktion • geografi • køn • alder • andre specielle studiebaser

  24. Associationsmål Case-kontrol undersøgelse, komplet follow-up • Stikprøve af raske • case – non-case design • Associationsmål • OR = a/c / b/d • OR ~ RR • sygdommen sjælden • RR tæt ved 1,0

  25. Case-kontrol undersøgelse, komplet follow-up Associationsmål • Stikprøve af studiebase – personer i alt • case-base design • kumulativ incidens proportions sampling • Associationsmål • ”OR” ~ RR • uafhængig af sygdomshyppighed • uafhængig af størrelsen af RR

  26. Associationsmål Case-kontrol undersøgelse, komplet follow-up • Estimerings- og vurderingsproblemer • cases i referencegruppen • statistiske metoder • Referencegruppen kan bruges i flere undersøgelser med • forskellige endpoints

  27. Case-kontrol undersøgelse Inkomplet follow-up (censureringer) Associationsmål • Stikprøve af studiebasen – observationstid • case-base design • incidence density sampling • hver gang en case opstår samples reference • personer fra den aktuelle studiebase

  28. Associationsmål Case-kontrol undersøgelse Inkomplet follow-up (censureringer) • Associationsmål • ”OR” ~ IRR • uafhængig af sygdomshyppighed og størrelse af IRR • Estimerings- og vurderingsproblemer • cases i referencegruppen • statistiske metoder • Referencegruppen er specifik for den pågældende undersøgelse

  29. Case-kontrol undersøgelse i follow-up undersøgelse Ikke økonomisk at gennemføre en follow-up undersøgelse, men heller ikke mulig at indhente relevante eksponerings- oplysninger på det tidspunkt, hvor cases og referencepersoner kan identificeres. • N-3 fedtsyrer i 30. graviditetsuge

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