epidemiological study designs.PPpresentationsX

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1. EPIDEMIOLOGY Epidemiological study designs Sewnet Getaye ( BSC,MPH) Email: [email protected] 1

2. Session Objectives After completing this session learners will be able to Understand epidemiological study designs Differentiate the type of study design used by investigators Identify the merits and demerits of each study designs Apply epidemiological study designs to conduct a study.

3. Study design Definition A study design is a specific plan or protocol for conducting the study, which allows the investigator to translate the conceptual hypothesis into an operational one.

4. Types of Epidemiologic study designs Based on objective/focus/research question Descriptive studies Describe: who, when, where & how many 2. Analytic studies Analyse: How and why

5. Types of Epidemiologic study designs II. Based on the role of the investigator Observational studies The investigator observes nature No intervention Intervention/Experimental studies Investigator intervenes He has a control over the situation

6. Types of Epidemiologic study designs III. Based on timing 1. One-time (one-spot) studies Conducted at a point in time An individual is observed at once 2. Longitudinal (Follow-up) studies Conducted in a period of time Individuals are followed over a period of time

7. Types of Epidemiologic study designs IV. Based on direction of follow-up/data collection Prospective Conducted forward in time 2. Retrospective Conducted backward in time

8. Types of Epidemiologic study designs V. Based on type of data they generate Qualitative studies Generate contextual data Also called exploratory studies 2. Quantitative studies Generate numerical data Also called explanatory studies

9. Types of Epidemiologic study designs VI. Based on study setting Community-based studies Conducted in communities 2. Institution-based studies Conducted in institutions 3. Laboratory-based studies Conducted in major laboratories

10. Types of Epidemiologic study designs VII. Standard classification Cross-sectional studies Case-control studies Cohort studies Experimental studies

11. Classification of Study designs

12. Descriptive studies Relatively cheap in terms of time cost, because it can use information already collected. It usually describes: Who gets a disease and who doesn’t ( person) Where rates are highest and lowest ( place) Temporal patterns of disease (time)

13. Descriptive studies It is the most common form of study in medical literatures There are three forms of descriptive study designs Correlational /ecological design ( use population as study subject) Case report and/or case series studies Cross-sectional survey

14. Case report and case series Case reports describe the experience of a single patient Case series describe the experience of a group of patients with similar diagnosis Both document unusual occurrences 14

15. Case report It is the study of health profile of a single individual using a careful and detailed report by one or more clinicians It is made using a history, physical examination and lab investigation Report is usually documented if there is unusual medical occurrence, thus it may be first clue for identification of a new disease It is useful in constructing a natural history of individual disease. 15

16. Case report Eg. The first one AIDS cases in USA. One young, previously healthy homosexual man was diagnosed as having Pneumocysti carinii pneumonia at Los Angeles hospital in 1980. This form of pneumonia had been seen almost exclusively among older men and women whose immune systems were suppressed. This unusual circumstance suggested that this individual was actually suffering with a previously unknown disease, subsequently it was called AIDS 16

17. Case series First case report may stimulate compilation of additional case reports….a case series e.g. occurrence of Pneumocystis carinii on young, homosexual man with no history of immune deficiency stimulated the collection of additional similar cases Collection of case reports in a short period of time 17

18. Case series Describes characteristics of a number of patients with a given disease Similar to case report, it is usually made on cases having new and/ or unusual disease It is often used to detect the emergence of new disease or an epidemics 18

19. Case series Eg. The first five AIDS cases in USA. Five young, previously healthy homosexual men were diagnosed as having Pneumocysti carinii pneumonia at Los Angeles hospital during a six month period from 1980 to 1981. This form of pneumonia had been seen almost exclusively among older men and women whose immune systems were suppressed. This unusual circumstance suggested that these individuals were actually suffering with a previously unknown disease, subsequently it was called AIDS. 19

20. Case report / series Advantages : Use available clinical data Detailed individual data Useful for the recognition of new diseases Suggest need for investigation (hypothesis generation) Disadvantages : May reflect experience of one person or one clinician No explicit comparison group Difficult to compute rate as the denominator is not known 20

21. Ecological study Based on aggregate measures of exposure and outcome from several populations The units of analysis are populations or groups of people rather than individuals Correlations are obtained between exposure rates and disease rates among different groups or populations Compare populations in different countries at the same time or the same population on one country at different times 21

22. Ecological study Example Suppose air pollution is higher in Baltimore than in Tampa, but mortality from lung disease is lower in Baltimore than in Tampa. It would be fallacious to conclude that air pollution protects against lung disease deaths. It is possible that persons dying of lung disease in Tampa may have moved from cities with high air pollution or that another risk factor for lung disease – such as smoking – is more prevalent in Tampa than Baltimore. We do not know the cumulative exposures of cases and non-cases in either city. 22

23. Ecological study Ecologic Fallacy: Observations made at the group level may not represent the exposure-disease relationship at the individual level The ecologic fallacy occurs when incorrect inferences about the individual are made from group level data 23

24. Ecological study Ecological study Strength Easy to do Quick and inexpensive Often use already available information Generate hypotheses for additional study 24

25. Ecological study Limitation Unable to examine data for individuals; data on exposure and data on outcome are collected independently No assurance that persons with exposure (risk factor) of interest are the same ones with the outcome (disease) of interest Association at the aggregate level may not reflect association at the individual level - The ecologic fallacy 25

26. Cross-sectional studies In this study design information about the status of an individual with respect to presence/absence of exposure and diseased is assessed at a point in time. Cross-sectional studies are useful to generate a hypothesis rather that to test For factors that remain unaltered overtime (e.g. sex, race, blood group) it can produce a valid association

27. Cross-sectional studies Comparison groups are formed after data collection The object of comparison are prevalence of exposure or disease Groups are compared either by exposure or disease status Cross-sectional studies are also called prevalence studies (point) Cross-sectional studies are characterized by concurrent classification of groups

28. Cross-sectional studies

29. Cross-sectional studies

30. Design and analysis of cross-sectional studies

31. Design and analysis of cross-sectional studies

32. Design and analysis of cross-sectional studies

33. Design and analysis of cross-sectional studies Measure of association in cross-sectional studies: -Odds ratio

34. Cross-sectional studies Uses of Cross-Sectional Studies Hypothesis generation Estimation of the magnitude and distribution of a health problem Intervention planning

35. Cross-sectional studies Advantages of cross-sectional studies Less time consuming Less expensive Provides more information Describes well Generates hypothesis

36. Cross-sectional studies Limitations of cross-sectional studies Antecedent-consequence uncertainty “Chicken or egg dilemma” Data dredge leading to inappropriate comparison More vulnerable to bias

37. Example of Cross-sectional study A study was conducted among 240 people to determine the relationship between coffee drinking and stomachache. The people were asked if they had stomachache and if they were coffee drinkers. Out of 80 people who reported that they were coffee drinkers, 20 reported having stomachache. Of those who reported that they do not drink coffee 4 reported having stomachache

38. Example … Summarize the result by 2-by-2 table ? Calculate the prevalence of stomachache? Calculate the prevalence of coffee drinking ? Calculate OR?

39. Example … a. b. P= = =10% c. P= = = 33.3% d. OR= = = 13   Drank coffee Stomach ache Yes No Total Yes 20 60 80 No 4 156 160 Total 24 216 240

40. Analytical study designs Purpose / aim of analytic epidemiologic study designs To test hypothesis about causal relationship To search for cause & effect To compare treatment regimens/prevention programs To asses diagnostic tests To quantify the association b/n exposure & outcome (measure of association) It focus on determinants of the diseases by testing hypothesis

41. Case-control studies Subjects are selected with respect to the presence (cases) or absence (controls) of disease, and then inquiries are made about past exposure We compare diseased (cases) and non-diseased (controls) to find out the level of exposure Exposure status is traced backward in time

42. Case-control studies

43. Case-control studies In case-control study design the disease status is fixed by the investigator prior to the data collection while exposure status to the expected risk factor is random

44. Case-control studies

45. Case-control studies Features to look for in a Case-control study Is selection bias present? Are cases and controls similar in all important respects except the disease in question? Is information bias present? Is information about exposures gathered in the same way for cases and controls? Is confounding factors present? Could the results be accounted for by the presence of a factors associated with both the exposure and outcome but not directly involved in the causal pathway?

46. Case-control studies Steps in a case control study Define the study’s objectives Identify and select “cases”: Select “controls” Collect data: measure level of “exposure” in both cases and controls Analyze data: compare the exposure frequencies in those with and without disease

47. Case-control studies Choosing cases and controls in case-control study Ideally, cases are selected from a clearly defined population, often called the source population, and controls are selected from the same population that yielded the cases Control group should be representative of the population from which the cases are derived, typically called the source population

48. Case-control studies Choosing controls Two common types of controls are population based and hospital-based controls Population-based controls In population-based case-control study, controls are selected from the community An advantage is that cases and controls come from the same source population, so they are similar in some way A disadvantage is that it is difficult to obtain population lists and to identify and enroll subjects

49. Case-control studies Hospital-based controls In a hospital-based case-control study, controls are selected from hospital patients with illnesses other than the disease of interest An advantage is that hospital controls are easily accessible, tend to be cooperative, and are inexpensive A disadvantage is that hospital-based controls are not likely to be representative of the source population that produced the cases

50. Case-control studies

51. Case-control studies Recall information in Case-control study Information on some past exposures depends on memory of events from both cases and controls Recall bias occurs when the recall is better among cases than controls because of the presence of the disease Consequently, a false association may be found between exposure and disease

52. Case-control studies Case-control study Strengths Good for studying rare outcomes Can examine many exposures at the same time Quick & Cheap Provides Odds Ratio

53. Case-control studies Limitation Cannot measure Incidence, Prevalence and Relative Risk Can only study one outcome Not suitable for rare exposures Highly liable to selection bias and recall bias Difficult to establish temporal relationship of exposure-disease relationship i.e. Does not establish sequence of events

54. Example of Case-control study In a study examining the relationship between the chili pepper consumption and gastric cancer, the researchers sampled 910 study subjects, 213 with gastric cancer and 697 controls, and asked their prior history of chili pepper consumption. The researchers found that,204 and 552 of the total cases and controls had the history of chili pepper consumption respectively. a) Draw 2-by-2 crosstab? b) Calculate OR?

55. Example … a. b. OR= = =   Ate chili pepper Gastric cancer Yes No Total Yes 204 552 756 No 9 145 154 Total 213 697 910

56. Cohort study What is a cohort? A well-defined group of people who share a common characteristic/ experience followed-up for specified period of time Examples – Exposure cohort: individuals assembled as a group based on some common exposure e.g. asbestos exposure in ship yards 56

57. Cohort study Select group of disease-free people who are classified according to a specific exposure Follow them over time for a specified period of time to see the occurrence of outcomes of interest in groups Measure and compare the incidence of outcome between exposed and unexposed groups Subjects are sampled by their exposure status 57

58. Cohort study 58

59. Cohort study Directionality : Exposure to Outcome Temporal relationship between observations of exposure(E) and outcome(O) Cohort studies can be either Prospective Retrospective 59

60. Prospective cohort study 60

61. Prospective cohort study Investigator starts the study (from the beginning) with the identification of the population and the exposure status (exposed/not exposed groups) Follow them (over time) for the development of disease(future follow-up) Time frame of prospective cohort study: From present to future Takes relatively long time to complete the study 61

62. Prospective cohort study: Example 62

63. Prospective cohort study: Example Framingham Study Objectives : To study the impact of several factors on incidence of cardiovascular diseases Exposures : Blood pressure, blood cholesterol level, electrocardiogram abnormalities, smoking, body weight, diabetes, physical exercise, etc. Multiple Outcomes : Coronary heart disease, stroke, congestive heart failure, peripheral arterial disease 63

64. Prospective cohort study: Example The study started with a defined population There were several hypotheses to be tested -Different exposures and different outcomes For each exposure, investigators identified the “exposed "and “not exposed "groups For each exposure, the participants were followed for the development of disease Different exposures were studied, as well as different diseases 64

65. Timeline Milestones from the Framingham Study 1948:start of the Framingham Heart Study 1960:cigarette smoking found to increase risk of heart disease 1961:cholesterol level, blood pressure, and ECG abnormalities found to increase risk of heart disease 1967:physical activity found to reduce risk of heart disease; obesity to increase the risk 1970:high blood pressure found to increase the risk of stroke 1971:Framingham Offspring Study initiated to assess familial and genetic factors as determinants of coronary heart disease 65

66. Retrospective cohort study 66

67. Retrospective cohort study Investigator uses existing data collected in the past to identify the population and the exposure status (exposed/not exposed groups) Determine disease status at the present time (no future follow-up) Time frame of retrospective cohort study: From past to present 67

68. Is Prospective or Retrospective cohort study???????????? In an "outcomes" analysis of coronary bypass surgery, health services researchers identified charts of all patients diagnosed with three vessel disease at three major clinical centers during the past ten years. These patients are separated into those who initially were treated surgically and those who were initially treated medically. Aggregate results for mortality and a variety of other outcome variables were compiled for each group, to produce prognostic profiles for those initially treated medically vs. those who received immediate surgery. 68

69. Is Prospective or Retrospective cohort study???????????? The pediatricians wanted to determine if there is a difference in the incidence of croup between the babies who are breastfed and those who are not breastfed. They followed a group of babies who are breastfed for six months following birth compared to another group of babies who are not breastfed for six months following birth. All the babies were free of croup when the pediatricians started the study. Of 100 breastfed babies, 2 babies developed the croup while of 50 babies who are not breastfed, 5 developed the croup. 69

70. Cohort study Features to look for in a cohort study Is selection bias present? Are the exposed and unexposed groups similar in all important respects except the exposure status? What steps were taken to minimize information bias? Is the outcome identified in the same way for both groups? 70

71. Cohort study How complete was the follow-up of both groups? What effort were made to limit loss to follow-up? Was loss to follow-up similar in both groups? Were potential cofounding factor present and controlled in the analysis? Did the investigators anticipated and gather information on potential confounding factors? What method(s) were used to assess and control confounding? 71

72. Cohort study Steps in a cohort study Define the study’s objectives Identify the study cohort Measure level of exposure and categorize cohort into “exposed” and “unexposed” groups Follow groups Measure outcome or “disease” (dependent) variable Analyze data: compare incidence rates in the different exposure groups 72

73. Cohort study 73

74. Example of Cohort study Researchers conducted a 20 year prospective cohort study to assess a possible association between eating fish and heart disease. They found that, of the 200 subjects who ate fish, 10 developed heart disease while of the 400 subjects who did not eat fish, 10 developed heart disease. Draw 2-by-2 table? Calculate RR? 74

75. Example … a. b. RR = = =   Ate fish Heart disease Yes No Total Yes 10 190 200 No 10 390 400 Total 20 580 600

76. Cohort study Strengths Can directly measure incidence rates Establishes time sequence for causality Eliminates recall bias Allows for accurate measurement of exposure variables Best for studying rare exposures Can measure multiple outcomes at the same time Can adjust for confounding variables Can calculate relative risk 76

77. Cohort study Limitation Expensive Time consuming Relatively poor for studying rare diseases or outcomes Selection bias Exposure may change over time Disease may have a long pre-clinical phase Potential for loss of study subjects to follow-up May require large study groups 77

78. Experimental study design A study in which a population is selected for a planned trial of a regimen, whose effects are measured by comparing the outcome of the regimen in the experimental group versus the outcome of another regimen in the control group Such designs are differentiated from observational designs by the fact that there is manipulation of the study factor (exposure), and randomization (random allocation) of subjects to treatment (exposure) groups 78

79. Experimental study design 79

80. Experimental study design 80

81. Experimental study design The investigator assigns the exposure variable ß Prospective in nature Randomized Clinical Trial(RCT): individual study subject Community Intervention Trial(CIT): study subject is the “community” 81

82. Experimental study design Why performed? Provide stronger evidence of the effect (outcome) compared to observational designs, with maximum confidence and assurance Yield more valid results, as variation is minimized and bias controlled Determine whether experimental treatments are safe and effective under “controlled environments” (as opposed to “natural settings” in observational designs) 82

83. Experimental study design Steps in an Experimental study Define the study’s objectives Identify the target population Recruit volunteers from the target population Random allocation of volunteers into two groups (assign study subjects to exposure and non-exposure group) 83

84. Experimental study design Assign exposure (independent) variable. Consider blinding Follow groups prospectively Measure outcome (dependent) variable Analyze data: compare incidence rates in exposed (treated) and unexposed (untreated) 84

85. Experimental study design Time frame of Experimental study: From present to future Random allocation Blinding The most serious objections to Experimental studies are ethical issues 85

86. Experimental study design 86

87. Experimental study design Strengths: Best measure of causal relationship Best design for controlling bias and confounding Can measure multiple outcomes Limitation : High cost and time-consuming Ethical issues may be a problem 87

88. Example of Experimental study Researchers are interested in determining whether the advice given during routine physical examination is effective in encouraging patients to stop smoking. In a study of current smokers, one group of patients was given a brief talk about hazards of smoking and was encouraged to quit. A second group received no advice pertaining to smoking. All the patients were given a follow up examination/assessing. In a sample of 114 patients who received advice, 11 reported, they had quit smoking and in a sample of 96 patients who had not received advice, 7 quit smoking. 88

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