1. 1. RETROSPECTIVE COHORT STUDY Dr.Bharat Kalidindi MPH 1st Sem Padmashree School of Public Health. Bangalore
2. 2. WHAT IS COHORT A cohort is any group of people who are linked in some way or the other. A cohort is a group of subjects who have shared a particular event together during a particular time span OR OR A cohort is a group of people who share a common characteristic or experience within a defined period
3. 3. EXAMPLE People born in India between 1918 and 1939. Survivors of an bus accident. Truck drivers who smoked between age 30 and 40. People who are exposed to a drug or vaccine or pollutant.
4. 4. WHAT IS A COHORT STUDY A cohort study is a form of longitudinal study (a type of observational study) used in medicine, social science, actuarial science, business analytics, and ecology. In a cohort study, a group of individuals exposed to a putative risk factor and a group who are unexposed to the risk factor are followed over time (often years) to determine the occurrence of disease. The incidence of disease in the exposed group is compared with the incidence of disease in the unexposed group.
5. 5. Defined population Begin with Exposed Not Exposed Develop Disease Do not Develop Disease Do not Develop Disease Develop Disease Identify Follow
6. 6. Cohort studies may be prospective or retrospective. Prospective cohort study (concurrent cohort study or longitudinal study): Subjects have been followed up for a period and the outcomes of interest are recorded. Retrospective cohort study (Non-concurrent cohort or historical cohort study): Study done after the occurrence of both the exposure and outcome.
7. 7. AN AMBIDIRECTIONAL COHORT STUDY A cohort study may also be ambidirectional , meaning that there are both retrospective and prospective phases of the study. Ambidirectional studies are much less common than purely prospective or retrospective studies, but they are conceptually consistent with and share elements of the advantages and disadvantages of both types of studies
8. 8. The Air Force Health Study (AFHS) - also known as the Ranch Hand Study - was initiated by the U. S. Air Force in 1979 to assess the possible health effects of military personnel's exposure to Agent Orange and other chemical defoliants sprayed during the Vietnam War. The study was conducted comparing: 1,098 pilots exposed to dioxin in Vietnam (Operation Ranch Hand) 1,549 men who flew cargo missions in Southeast Asia during the same time
9. 9. ANALYSIS OF COHORT STUDIES Analysis of a cohort study uses either the risk or the rate ratio of disease in the exposed cohort compared with the rate or risk in the unexposed cohort. Rate Ratio = Incidence rate in exposed group (r1) Incidence rate in unexposed group (r0)
10. 10. FIRST, IDENTIFY EXPOSED a+b NOT EXPOSED c+d TOTAL
11. 11. THEN, FOLLOW TO SEE WHETHER EXPOSED a b a+b NOT EXPOSED c d c+d TOTAL Disease does not develop Disease develops
12. 12. CALCULATE AND COMPARE EXPOSED a b a+b a/a+b NOT EXPOSED c d c+d c/c+d Disease develops Disease does not develop TOTAL Incidence of disease a/a + b = Incidence in exposed c/c + d = Incidence in not exposed
13. 13. WHEN IS A COHORT STUDY WARRANTED? When the (alleged) exposure is known When exposure is rare and incidence of disease among exposed is high (even if the exposure is rare, determined investigators will identify exposed individuals) When the time between exposure and disease is relatively short When adequate funding is available When the investigator has a long life expectancy
14. 14. STRENGTHS AND WEAKNESSES OF COHORT STUDIES Strengths: Multiple outcomes can be measured for any one exposure. Can look at multiple exposures. Exposure is measured before the onset of disease (in prospective cohort studies). Good for measuring rare exposures, for example among different occupations. Demonstrate direction of causality. Can measure incidence and prevalence.
15. 15. Weaknesses: Costly and time consuming. Prone to bias due to loss to follow-up. Prone to confounding. Participants may move between one exposure category. Knowledge of exposure status may bias classification of the outcome. Being in the study may alter participant's behaviour. Poor choice for the study of a rare disease. Classification of individuals (exposure or outcome status) can be affected by changes in diagnostic procedures.
16. 16. RETROSPECTIVE STUDY In a retrospective study, the outcome of interest has already occurred at the time the study is initiated. A retrospective study design allows the investigator to formulate ideas about possible associations and investigate potential relationships, although causal statements usually should not be made.
17. 17. An investigator conducting a retrospective study typically utilizes administrative databases, medical records, or interviews with patients who are already known to have a disease or condition. In general, the reasons to conduct a retrospective study are to: Study a rare outcome for which a prospective study is not feasible. Quickly estimate the effect of an exposure on an outcome. Obtain preliminary measures of association. A retrospective cohort study allows the investigator to describe a population over time or obtain preliminary measures of association to develop future studies and interventions.
18. 18. Defined population Exposed Not Exposed Develop Disease Do not Develop Disease Do not Develop Disease Develop Disease 1989 2015
19. 19. The investigators jump back in time to identify a cohort of individuals at a point in time before they have developed the outcomes of interest, and they try to establish their exposure status at that point in time. They then determine whether the subject subsequently developed the outcomes of interest. The Distinguishing feature of a retrospective cohort study is that the investigators conceive the study and begin identifying and enrolling subjects after outcomes have already occurred.
20. 20. Suppose investigators wanted to test the hypothesis that working with the chemicals involved in tire manufacturing increases the risk of death. Since this is a fairly rare exposure, it would be advantageous to use a special exposure cohort text annotation indicator such as employees of a large tire manufacturing factory. The employees who actually worked with chemicals used in the manufacturing process would be the exposed group, while clerical workers and management might constitute the "unexposed" group. However, rather than following these subjects for decades, it would be more efficient to use employee health and employment records over the past two or three decades as a source of data. In essence, the investigators are jumping back in time to identify the study cohort at a point in time before the outcome of interest (death) occurred. They can classify them as "exposed" or "unexposed" based on their employment records, and they can use a number of sources to determine subsequent outcome status, such as death
21. 21. Retrospective cohort studies like this are very efficient for studying rare or unusual exposures, but there are many potential problems here. Sometimes exposure status is not clear when it is necessary to go back in time and use whatever data is available, especially because the data being used was not designed to answer a health question. Even if it was clear who was exposed to tire manufacturing chemicals based on employee records, it would also be important to take into account (or adjust for) other differences that could have influenced mortality, i.e., confounding factor. For example, it might be important to know whether the subjects smoked, or drank, or what kind of diet they ate. However, it is unlikely that a retrospective cohort study would have accurate information on these many other risk factors.
22. 22. Investigator: Uses existing data collected in the past to identify the population and the exposure status (exposed/not exposed groups). Determines at present the (development) status of disease. Investigator spends a relatively short time to: Assemble study population (and the exposed/not exposed groups) from past data. Determine disease status at the present time (no future follow-up).
23. 23. DISADVANTAGES OF RETROSPECTIVE COHORT STUDIES As with prospective cohort studies, they are not good for very rare diseases. If one uses records that were not designed for the study, the available data may be of poor quality. There is frequently an absence of data on potential confounding factors if the data was recorded in the past. It may be difficult to identify an appropriate exposed cohort and an appropriate comparison group. Differential losses to follow up can also bias retrospective cohort studies.
24. 24. SELECTION BIAS RETROSPECTIVE COHORT STUDIES Bias can occur in retrospective cohort studies if subjects in one of the exposure groups are more or less likely to be selected if they had the outcome of interest. Ex: Retrospective cohort study with the aim of measuring the association between exposure to an industrial solvent and risk of death. study was conducted using employee health records for the past 20 years from a large company. Some of the employees were exposed to the solvent, but others were not. During this span of time, some employee health records were lost, but they were more likely to retain the records of employees who had been exposed to the solvent and then subsequently died.
25. 25. real risk ratio would have been found to be 2.0, if all records had been retained. as a result of selective retention of records for exposed workers who died prematurely, the apparent risk ratio was 2.42, i.e. an overestimate of the association.
26. 26. OUTBREAK OF GIARDIA Occurred in Milton The request for assistance was made some time after the start of the outbreak, and the outbreak was winding down by the time DPH began their study. The outbreak was clearly concentrated among members of the Wollaston Golf Club in Milton, MA , which had two swimming pools, one for adults and a wading pool for infants and small children. The investigators thought that contamination of the kiddy pool by a child shedding Giardia into their stool was the most likely source. Investigators knew the denominators for the exposure groups, so they could calculate the cumulative incidence, risk difference, and the risk ratio. People who had spent time in the kiddy pool had 9.0 more cases per 100 persons than those who spent time in the kiddy pool. The risk ratio was 3.27.
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