cohort study epidemiology Study design.pptx

Cohort study epidemiology

1. Cohort studies

2. Topics covered Definition Design and steps in conducting cohort  Types of Cohort Study Methodological Issues Advantages and disadvantages of Cohort study

3. Definition

4. The modern epidemiological definition of the word now means a “group of people with defined characteristics who are followed up to determine incidence of, or mortality from, some specific disease, all causes of death, or some other outcome.”

5. A well-designed cohort study can provide powerful results. In a cohort study, an outcome or disease-free study population is first identified by the exposure or event of interest and followed in time until the disease or outcome of interest occurs   It has a temporal framework to assess causality and thus have the potential to provide the strongest scientific evidence.

6. Example You are examining the relationship between exposure to pesticides and the incidence of a diagnosis of Parkinson’s disease. You recruit a group of healthy participants, all of whom were free of Parkinson’s disease at the beginning of your study. You then collect data on their exposure to pesticides over time, tracking incidences of Parkinson’s disease. After several years, your results conclude that those who were exposed to higher levels of pesticides had a higher risk of developing Parkinson’s disease compared to those who were not.

7. Design and Steps in conducting a cohort study   1.Define a source population 2.Select Study Populations (subjects & controls):  Two methods: based on exposure status OR based on factor other than exposure e.g. geographic location  3.Measure the exposure  4.Follow up at intervals to get accurate outcome data  5.Analyze data.

8. Types: Cohort studies can be: 1.Prospective  2.Retrospective

9. Prospective cohort study Prospective studies are carried out from the present time into the future. Because prospective studies are designed with specific data collection methods

10. Prospective...

11. Exemplified by :  the landmark Framingham Heart Study, started in 1948 and still ongoing . In the plastic surgery literature this study design is generally seen to be inefficient and impractical.

12. Retrospective cohort study Retrospective cohort studies, also known as historical cohort studies, are carried out at the present time and look to the past to examine medical events or outcomes. 

13. In other words, a cohort of subjects selected based on exposure status is chosen at the present time, and outcome data (i.e. disease status, event status), which was measured in the past, are reconstructed for analysis. . 

14. The primary disadvantage of this study design is the limited control the investigator has over data collection. The existing data may be incomplete, inaccurate, or inconsistently measured between subjects.  However, because of the immediate availability of the data, this study design is comparatively less costly and shorter than prospective cohort studies

15. Methodological Issues   1.Selection of Subjects in Cohort Studies  The hallmark of a cohort study is defining the selected group of subjects by exposure status at the start of the investigation.  •Characteristic of subject selection is to have both the exposed and unexposed groups be selected from the same source population 

16. Subjects who are not at risk for developing the outcome should be excluded from the study. The source population is determined by practical considerations, such as sampling 

17. 2. Attrition Bias (Loss to follow-up): Because prospective cohort studies may require long follow-up periods, it is important to minimize loss to follow-up.  Loss to follow-up is a situation in which the investigator loses contact with the subject, resulting in missing data.

18. If too many subjects are loss to follow-up, the internal validity of the study is reduced. A general rule of thumb requires that the loss to follow-up rate not exceed 20% of the sample

19. Any systematic differences related to the outcome or exposure of risk factors between those who drop out and those who stay in the study must be examined, if possible, by comparing individuals who remain in the study and those who were loss to follow-up or dropped out. 

20. Methods to minimize loss to follow-up Obtain information to allow future tracking. Collect subject's contact information (e.g. mailing addresses, telephone numbers, and email addresses)  Collect social security and/or Medicare numbers  Maintain periodic contact  1.By telephone: may require calls during the weekends and/or evenings  2. By mail: repeated mailings by e-mail or with stamped, self-addressed return envelopes 

21. Advantages and disadvantages of cohort studies: Advantages Incidence, can be calculated.  Several possible outcomes related to exposure can be studied simultaneously.  It provides a direct estimate of relative risk.  Dose response ratios can also be calculated.  Since comparison groups are formed before disease develops, certain forms of bias can be minimized like mis-classification.

22. Disadvantages It takes a long time to complete the study and obtain results. It is unusual to lose a substantial proportion of the original cohort. There may be changes in the standard methods or diagnostic criteria of the disease. Selection of comparison groups which are representative of the exposed and unexposed segments of the population is a limiting factor. Can Be expensive and time consuming  It involves a large number of people

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