Analytical epidemiology cross sectional and cohort
- 1. ANALYTICAL EPIDEMIOLOGY Cross Sectional & Cohort Study BY: JAGAN KUMAR OJHA MSc. Tutor, SNC, Bhubaneswar
- 2. ANALYTICAL STUDY Observational studies are those in which individuals are observed and their outcomes are measured by the investigators. Interventional studies are those in which the research subjects are assigned by the investigator to a treatment or other intervention, and their outcomes are measured.
- 3. ANALYTICAL STUDY The purpose of an analytic study in epidemiology is to identify and quantify the relationship between an exposure and a health outcome. The hallmark of such a study is the presence of at least two groups, one of which serves as a comparison group.
- 4. Cross-sectional Studies A cross-sectional studies A type of observational study The investigator has no control over the exposure of interest (e.g. diet). It involves Identifying a defined population at a particular point in time Measuring a range of variables on an individual basis e.g. include past and current dietary intake At the same time measuring outcome of interest e. g. obesity
- 5. Measurement of exposure of interest and outcome of interest is carried out at the same time (e.g. Obesity and Hypertension) Cross-sectional studies deals with the situation existing at a given time (or during a given period) in a group or population These may be concerned with: • The presence of disorders such as diseases, disabilities and symptoms of ill health. • Factors Associated with health & disease such as exposure to specific environmental exposure or defined behavioral attributes and demographic attributes. Cross-sectional Studies
- 6. When the investigator draws a sample out of the study population of interest, and examines all the subjects to detect those having the disease / outcome and those not having this outcome of interest. At the same time finds out whether or not they have the presence of the suspected cause (exposure) is called the Cross sectional analytic study. Cross-sectional Studies
- 7. Cross-sectional studies May be –Descriptive –Analytical – Or Both In descriptive CS Study, it yields information about a single variable, or about each of number of separate variables in a study population. In analytic CS study, data on the prevalence of both exposure and a health outcome are obtained for the purpose of comparing health outcome i.e. differences between exposed and unexposed. It provides information about the presence and strength of associations between variables, permitting testing of hypothesis. Cross-sectional Studies
- 11. Synonyms- oInstantaneous study oPrevalence study oSimultaneous study Cross-sectional Studies
- 13. o Correlation, regression coefficient, rate difference, mean difference etc.
- 15. Advantages & Disadvantages Cheap and quick studies. Data is frequently available through current records or statistics. Ideal for generating new hypothesis. Correlation between two continuously distributed phenomenon can be studied. Starting point for a cohort study. Needs large sample size. Large number of logistic support needed. The importance of the relationship between the cause and the effect cannot be determined. Temporal weakness: • Cannot determine if cause preceded the effect or the effect was responsible for the cause. A D V D I S A D V
- 16. Cohort Study In epidemiology, the term “cohort” is used to define a set of people with a common characteristics followed for a certain period of time. A cohort may be nurses,10-19 year adolescence, college students, Antenatal mother etc. Cohort studies are a type of medical research used to investigate the causes of disease and to establish links between risk factors and health outcomes. So these types of studies look at groups of people. They can be forward-looking (prospective) or backward- looking (retrospective).
- 17. Cohort Study Types of Cohort Studies Concurrent Cohort Study (prospective or longitudinal) Retrospective Cohort Study (historical cohort or non- concurrent prospective study) Both designs are identical…comparing exposed and non- exposed populations The only difference is calendar time.
- 18. Cohort Study • Prospective cohort studies are planned in advance and carried out over a future period of time. • Retrospective cohort studies look at data that already exist and try to identify risk factors for particular conditions.
- 19. Cohort Study • Prospective: none of the subjects have the disease (or other outcome) being measured when the study commences; data analysis happens after a period of time has elapsed. • Retrospective (Historical): the researcher looks at historical data for a group. Some of the people in this group have developed the disease, and some have not. This can result in finding out who has the disease and when they developed it.
- 20. Elements of a cohort study Selection of study subjects Obtaining data on exposure Selection of a comparison group Follow up Analysis
- 21. Prospective cohort Study The investigators identify the subjects as a cohort, and collect baseline exposure data on all subjects, before any of the subjects have developed any of the outcomes of interest. Similarly the non- exposure group from the study subjects are selected as comparison group. The subjects are then followed into the future in order to record the development of any of the outcomes of interest. The follow up can be conducted by mail questionnaires, by phone interviews, physical examinations, and laboratory or imaging tests.
- 24. Retrospective Cohort Study • In this study, the investigator select a cohort in which some of the participants already have a known disease or outcome. The study looks back into the past to try to determine why the participants have the disease or outcome and when they may have been exposed. • In a retrospective cohort study the researcher: Uses historical data to identify members of a population who have been exposed (or not exposed) to a disease or outcome.
- 27. • Prospective example: a group of 100 people with high risk factors for AIDS are followed for 20 years to see if they develop the disease. Another group of 100 people who have low risk factors are also followed as comparison group. • Retrospective example: From a group of transgender population, a group of 100 people with AIDS might be asked about their lifestyle choices and medical history in order to study the origins of the disease. A Second group of 100 people without AIDS are also studied and the two groups are compared.
- 28. Exercise How would you design a cohort study of the association between preterm delivery and cigarette smoking? • An exposed and a non-exposed group would first be identified e.g. women presenting for prenatal care would be classified by smoking status – smokers and non-smokers. • These women would be followed to determine whether or not preterm delivery occurred. The rates of the preterm delivery would be compared among the smokers and non-smokers.
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