Analytical study designs.pptx

Observational study is divided into descriptive and analytical studies.

Non-experimental

Observational because there is no individual intervention

Treatment and exposures occur in a “non-controlled” environment

Individuals can be observed prospectively or retrospectively

COHORT STUDY- an “observational” design comparing individuals with a known risk factor or exposure with others without the risk factor or exposure.

looking for a difference in the risk (incidence) of a disease over time.

best observational design

data usually collected prospectively (some retrospective)

CASE CONTROL - EFFECT TO CAUSE

Retrospective

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1. ANALYTICAL STUDY DESIGNS - Dr. ARYASREE L.

2. Overview Cohort study Case control study

3. Observational Studies Non-experimental Observational because there is no individual intervention Treatment and exposures occur in a “non-controlled” environment Individuals can be observed prospectively or retrospectively

4. Figure 9-3 Selection of study groups in experimental and observational epidemiologic studies .

5. Basic Questions in Analytic Epidemiology Look to link exposure and disease What is the exposure? Who are the exposed? What are the potential health effects? What approach will you take to study the relationship between exposure and effect?

6. COHORT STUDY

7. Cohort Studies an “observational” design comparing individuals with a known risk factor or exposure with others without the risk factor or exposure. looking for a difference in the risk (incidence) of a disease over time. best observational design data usually collected prospectively (some retrospective)

8. Figure 9-4 Design of a cohort study beginning with exposed and nonexposed groups. Downloaded from: StudentConsult (on 26 February 2013 06:04 AM) © 2005 Elsevier Design of cohort study

9. Figure 9-5 Design of a cohort study beginning with a defined population. Downloaded from: StudentConsult (on 26 February 2013 06:04 AM) © 2005 Elsevier

10. Cohort design Time Study begins here Study Population Free of Disease Factor Present Factor Absent Disease No disease Disease No disease Present Future

11. Prospective Study - looks forward, looks to the future, examines future events, follows a condition, concern or disease into the future time Study begins here

12. Retrospective Study - “to look back”, looks back in time to study events that have already occurred time Study begins here

13. Figure 9-7 Time frame for a hypothetical retrospective cohort study begun in 2008. Downloaded from: StudentConsult (on 26 February 2013 06:04 AM) © 2005 Elsevier

14. Figure 9-8 Time frames for a hypothetical prospective cohort study and a hypothetical retrospective cohort study begun in 2008. Downloaded from: StudentConsult (on 26 February 2013 06:04 AM) © 2005 Elsevier

15. The Framingham study

16. Aniline dyes and urinary bladder cancer

17. Elements of cohort study Selection of study populations Gathering baseline information Follow up Analysis

18. 1. Selection of study population General population Special exposure cohorts

19. 2. Gathering baseline information Valid assessment of exposure status Exclude who are having disease of interest Data on other risk factors

20. 3. Choice of comparison group Internal comparison group External comparison group

21. 4. Follow up Uniform and complete follow up Complete assessment of exposures and outcomes Standardized diagnosis of outcomes

22. Presentation of the data in a cohort study in a 2x2 table

23. Relative Risk The relative risk can be defined as the probability of an event (developing a disease) occurring in exposed people compared with the probability of the event in unexposed people, or as the ratio of these two probabilities. RR= Risk in exposed Risk in unexposed

24. Interpreting relative risk of a disease If RR =1 Risk in exposed equal to risk in unexposed(no association) If RR >1 Risk in exposed greater than risk in unexposed(positive association;possibly causal) If RR < 1 Risk in exposed less than risk in unexposed (negative association; possibly protective)

25. Cohort study strengths and weaknesses Strengths weaknesses Allows calculation of incidence Long calendar time Examine multiple outcomes for a given exposure Not good for rare diseases Clarity of temporal sequence Not good for diseases with a long latency Good for investigating rare exposures Differential loss to follow up can introduce bias

26. Case control

27. Elements of case control study Selection of cases Selection of controls Information on exposure Analysis

28. 1. Selection of cases All people in source population who develop the disease of interest Clear definition of outcome studied Prevalent vs incident cases

29. Sources of cases Hospital /clinic based cases Population based

30. 2. Selection of controls Population based Health care facility based Case based

31. 3. Collecting good data on exposure Objectively – reproducibility of exposure measurement Accurately – information reflecting as closely as possible the effect of exposure Precisely – Quality management in exposure measurement

32. Presentation of the data of a case control study in a 2x2 table

33. Odds ratio An odds ratio (OR) is a measure of association between an exposure and an outcome. The OR represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure.

34. Interpreting odds ratio OR=1 Odds of exposure among cases and controls are same Exposure is not associated with disease OR>1 Odds of exposure among cases are higher than controls Exposure is positively associated with disease OR<1 Odds of exposure among cases are lower than controls Exposure is negatively associated with disease

35. Case control study strengths and weaknesses STRENGTHS WEAKNESSES Good for rare outcomes Susceptible to recall bias Relatively quick to conduct Selection of an appropriate comparison group may be difficult Requires comparatively few subjects Rates of disease in exposed and unexposed individuals cannot be determined Multiple exposures or risk factors can be examined

36. Example 1 Thyroid hormones, namely triiodothyronine (Free T3), thyroxine (Free T4) and thyroid stimulating hormone (TSH) were evaluated at the time of diagnosis of preeclampsia in 82 pregnant women and equal number of matched controls. (Kumar et al.) Case Control

37. Example 2 46,112 never users of oral contraception and women 819,175 ever users were followed for 39 years to ascertain mortality risk. (Hannaford et al.) Cohort

38. Example 3 275 women attending the antenatal clinic at Kilifi district hospital, Kenya, were recruited in November 1993 and tested for malaria in order to calculate the prevalence. (Shulman et al.) Cross-Sectional

39. Example 4 270 wards randomised to 3 groups of 90 each for women to receive weekly a single oral supplement of placebo, vitamin A or â carotene for over 3.5 years and followed to determine pregnancy-related mortality. (West et al.) Clinical Trial

40. Example 5 A survey among second trimester pregnant women 18-44 took place between April 2003 and November 2003 to determine the prevalence of anemia and hookworm. ( Larocque et al.) Cross-Sectional

41. Example 6 431 women were enrolled in a study within 21 days of conception and monitored throughout pregnancy to determine caffeine exposure and pregnancy outcome. (Mills et al.) Cohort

42. References BCBR lecture 6 Gordis epidemiology Edition 6 National library of medicine

43. THANKYOU