study designs in epidemiology.. (1).pptx

Epidemiology

1. STUDY DESIGNS Richard Malumba

2. Lecture objectives How do we design and conduct studies to explain the etiology of and risk factors for human disease?

3. Intro.. Epidemiological study designs are broadly divided into observational and Experimental studies Observational studies are further divided into; Descriptive Studies Case reports/studies/series Ecological studies Cross-sectional studies Analytical Studies Case-Control Studies (retrospective) Cohort Studies (prospective/longitudinal)

4. Cont.. Experimental Studies Clinical Trials (RCTs) Community Trials

5. Case Reports and Case Series One of the most common and early origins of medical research conducted through careful observations by physicians and other health care providers of what they see during their clinical practice Such individual-level observations can be documented in a case report, describing a particular clinical phenomenon in a single patient, O r in a case series that describes more than one patient with similar problems

6. Cont.. The main objective of case reports and case series is to provide a comprehensive and detailed description of the case(s) under observation This allows other physicians to identify and potentially report similar cases from their practice, especially when they share geographic or specific clinical characteristics case reports and case series are merely descriptive in nature with no reference group to make a strict comparison

7. Advantages and Disadvantages Case reports and case series are key hypothesis-generating tools, especially when they are simple, inexpensive, and easy to conduct in the course of busy clinical settings However , the lack of a comparison group is a major disadvantage Furthermore , external validity (generalizability) is limited, given the biased selection of cases (all identified in clinical practice )

8. Cross-sectional study design Cross-sectional studies are mainly used for examining the characteristics of populations so as to detect associations with events of interest The relationship between exposure and disease is assessed simultaneously e.g., we may want to associate high cholesterol levels to increased risk of Congenital Heart Disease. In this case, we assess for congenital heart disease and cholesterol

9. Cont.. The general design of a cross-sectional (or prevalence) study follows the sequence that; We define a population and determine the presence or absence of exposure and the presence or absence of disease for each individual at the same time both exposure and disease outcome are determined simultaneously for each study participant views a snapshot of the population at a certain point in time in relation to exposure and disease

10. Challenges in using cross-sectional studies Cross-sectional studies identify prevalent other than incident disease. If there is an association therefore, it is more related to survival with the disease In addition, it is not possible to establish a temporal relationship – we cannot tell what preceded the other: the heart disease or the cholesterol

11. Case-Control Studies Suppose you are a clinician and you have seen a few patients with a certain disease You observe that many of them have been exposed to a particular agent— biological or chemical You hypothesize that their exposure is related to their risk of developing this disease How would you go about confirming or refuting your hypothesis?

12. Cont.. Case Control studies are “observational” studies The investigator does not actively intervene in the exposure-outcome process, but simply observes the process and then goes on to analyze those observations so as to make rational inferences

13. Cont.. In case control studies the relationship between cases of a particular disease with a possible exposure is investigated In this study-design the investigator goes through the following steps: Identifying persons with the disease of interest i.e. the cases: the starting point is the people with the disease Identify a comparable group of people without the disease who are the “control” group Establishing history of previous exposure to the suspected agent in both the cases and the controls Comparing exposure in the 2 groups

14. Cont.. The investigation process in case-control studies is retrospective. This is so because the process begins with identifying cases of a disease and then proceeds with inquiries about past exposure to the suspected causative agent In the analysis of case-control studies we compare past exposure to the suspected agent between cases and controls We compare the proportion of cases that were exposed to the proportion of controls that were exposed

15. Cont.. Inquiries about past exposure are through interviews, questionnaires, review of past medical and other records (e.g. employee records) and results of chemical/biological assays e.g. of blood and urine Odds ratios are a measure of association used

16. Advantages They can be conducted quickly. This means that they can provide quick answers to explain current problems They are usually cheap compared to other types of studies Since there is no follow up period, study subjects cannot be lost They are suited to investigating rare conditions

17. Disadvantages Does not establish sequence of events Potential for recall bias; either limitations in recall (forgetting), recall bias (cases may be more keen to recall historical exposures more than controls) or reporting bias (deception)

18. Cohort study design It is an observational and analytical study design that is very often used to test certain hypotheses It is used to test the relationship between certain risk factors or exposures and disease In Epidemiology, a cohort is a group of people who are defined by certain attributes like age, residence, occupation, etc. whom are followed up in time and in whom a record of events e.g. disease or death is noted

19. Why cohort study design? Usually done because epidemiologists want to test certain hypotheses of “causal associations” between a certain exposure/risk factor, and disease Such hypotheses are usually derived from other less rigorous studies in analytical sense like case reports or case series, or cross sectional studies that generate such hypotheses

20. Cont.. The cohort study design starts with Selection of groups of individuals; exposed and non-exposed individuals Follow up the groups in time to compare incidence of events in exposed vs non-exposed (the design may include two or more groups in each category

21. Features At the beginning of the study, the persons to be studied have not yet developed the disease of interest The exposure is known and can be measured in one way or another Two groups of people from the same population are followed up; one group has had or is under the exposure factor while the other group (the control group) is free of exposure factor. Both groups are followed up in time; both groups are initially disease free

22. Cont.. During follow-up, incident cases of disease are noted and comparisons made through calculations of risk of developing the disease in both groups By the very nature of this design, any suspected causal factor precedes the onset of disease and therefore this type of design is quite useful in proving causal relationship between exposure factor and disease

23. Advantages They are useful to investigate multiple effects of an exposure or exposures, over time e.g. One can test for example effect of smoking on the coronary vessels, as well as on the lungs over time, and can test the effects of smoking, H.T, Obesity on coronary heart diseases (CHD) Cohorts are very useful in measurements of time-relationships, relating exposure and time of onset of disease With cohorts, it is easy to calculate the risk of developing disease through the use of incidence rates

24. Disadvantages The nature of follow-ups makes the design quite expensive in a number of things: money, time and personnel Loss to follow-up of subjects is a very serious drawback in this design. This leads to bias if loss is not uniform in the two study group There is potential bias in assessing the outcome of exposure especially if such person doing the assessment knows the hypothesis under test and which subject was exposed and not exposed

25. Randomized Trials/RCTS Our objective, both in clinical practice and in public health, is to modify the natural history of a disease so as to prevent or delay death or disability and to improve the health of the patient or the population The challenge is to select the best available preventive or therapeutic measures to achieve this goal R andomized trials is considered the ideal design for evaluating both the efficacy and the side effects of new forms of intervention

26. Cont.. Randomized trials are experimental groups, which we follow up forward in time to detect incident events of interest and associate them to the exposure (intervention/ treatment) incidence of events of interest(outcome) in the exposed group is compared with that in the non-exposed We cannot intentionally expose a group of people to a putatively harmful substance (e.g. a suspected carcinogen) simply because we want to study an association The “exposure” in most randomized trials therefore is a “treatment” or “preventive measure”

27. Why RCTs? Evaluate new approaches to treatment and prevention, thus the concept of therapeutic and preventive trials. Evaluate tests of new health and medical care technology Assess new programs for screening and early detection of disease Assess new ways of organizing and delivering health care services

28. Forms of Randomized trials Clinical trial; a research activity or an experiment that involves the administration of a test regimen to humans to evaluate its efficacy and safety In this experiment the unit of randomization is the individual and the site of the trial is usually a clinic or a hospital ward

29. Cont.. C ommunity trial; an experiment in which the unit of allocation is a cluster of individuals or an entire community Clinical or community trials can be either therapeutic or preventive depending on the intervention being evaluated

30. Randomization The process of assigning trial participants to treatment group using an element of chance to determine the assignments in order to reduce bias Increases likelihood that the groups will be comparable with regard to baseline characteristics except the intervention under study

31. Blinding Concealing the allocation to the treatment groups is important so as to reduce bias in assessing treatment outcomes Masking or blinding can achieve concealing the allocation and use a code that is only broken after the analysis

32. Forms of blinding Open Label; Everyone i.e. the subjects, the investigators and the assessors know who is in which treatment group Single Blind; Either the subject or the investigators are unaware of the treatment group. Double Blind; Either the subjects and the investigators or the subjects and the assessors are unaware of the treatment group Triple Blind; Everyone i.e. the subjects, the investigators and the assessors are unaware of treatment group