Case control studies and cohort studies

Case control studies and cohort studies

1. CASE CONTROL STUDIES AND COHORT STUDIES Submitted by, Saharshan P M IV Z 8053 Nirmala College Muvattupuzha

2. COHORT STUDY A study design in which one or more cohorts are followed over a period of time to determine the incidence and natural history of a condition. The word “COHORT” means a group of people with defined characteristics who are followed up to determine incidence and causes of the outcome. As the study is conducted , Outcome from participants in each cohort is measured and relationships with specific characteristics determined. The studies may be prospective or retrospective

3. PROSPECTIVE COHORT STUDY It is carried out from the present to the future. A group of people is chosen who do not have the outcome of interest and the followed over a period of time to see whether they develop the outcome of interest. In this study design, the participants are enrolled into the study before they develop the disease or outcome Prospective cohort study is a research study that follows over time groups of individuals who are alike in many ways but differ by a certain characteristic (for example, female nurses who smoke and those who do not smoke) and compares them for a particular outcome (such as lung cancer). This study design is inefficient for investigating diseases with long latency periods and is vulnerable to a high loss to follow-up rate.

4. RETROSPECTIVE COHORT STUDIES It is carried out at the present time and look to the past In this design researchers enroll people who already have the disease/condition. In other words, a cohort of subjects selected based on exposure status is chosen at the present time, and outcome data ( ie . disease status, event status), which was measured in the past, are reconstructed for analysis. The cohort is “followed up” retrospectively One of the first recognized retrospective cohort studies was Lane- Claypon’s 1926 study of breast cancer risk factors. The study of 500 hospitalized patients and 500 controls led to the identification of most of the risk factors for breast cancer that we know today.

5. PROSPECTIVE VS. RETROSPECTIVE COHORT STUDY In a retrospective cohort study, the group of interest already has the disease/outcome. In a prospective cohort study, the group does not have the disease/outcome, although some participants usually have high risk factors. Retrospective example: 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. 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. A control group of 100 people who have low risk factors are also followed for comparison.

6. HOW TO RUN A COHORT STUDY Five steps in a cohort study: Identify the study subjects; ie . the cohort population Obtain baseline data on the exposure; measure the exposure at the start. Select a sub-classification of the cohort—the unexposed control cohort—to be the comparison group. Follow up; measure the outcomes using records, interviews or examinations. Do the data analysis where the outcomes are assessed and compared.

7. ADVANTAGES OF COHORT STUDY The use of cohorts is often mandatory as a randomised controlled trial may be unethical; for example, you cannot deliberately expose people to cigarette smoke. A single study can examine various outcome variables. For example, cohort studies of smokers can simultaneously look at deaths from lung, cardiovascular, and cerebrovascular disease. Cohorts permit calculation of the effect of each variable on the probability of developing the outcome of interest (relative risk-This is the ratio of the probability of developing the condition if exposed to a certain variable compared with the probability if not exposed). Retrospective studies are much cheaper as the data have already been collected. Gather data regarding sequence of events; can assess causality.

8. DISADVANTAGES OF COHORT STUDY Large numbers of subjects are required Susceptible to selection bias. Prospective Cohort Study May be expensive to conduct May require long durations for follow-up Susceptible to loss to follow-up Retrospective Cohort Study Susceptible to recall bias or information bias Less control over variables

9. CASE-CONTROL STUDIES A study that compares patients who have a disease or outcome of interest (cases) with patients who do not have the disease or outcome and looks back retrospectively to compare how frequently the exposure to a risk factor is present in each group. The goal is figure out the relationship between risk factors and disease or outcome and estimate the odds of an individual getting a disease or experiencing an event. This study involves the practice of interviewing patients about behaviours and conditions preceding illness. This study design was first recognized in Janet Lane- Claypon's study of breast cancer in 1926, revealing the finding that low fertility rate raises the risk of breast cancer.

10. HOW TO RUN A CASE-CONTROL STUDY Case-control studies have four main steps: The study begins by enrolling people who already have a certain disease or outcome. A second control group of similar size is sampled People are asked about their exposure to risk factors. Finally, an odds ratio (Odds of a condition occurring in exposed group divided by the odds of it occurring in non-exposed group) is calculated

11. ADVANTAGES OF CASE-CONTROL STUDY Good for examining rare outcomes or outcomes with long latency. Relatively quick to conduct. Relatively inexpensive. Existing records can be used. Requires comparatively few subjects.

12. DISADVANTAGES OF CASE-CONTROL STUDY • Susceptible to recall bias or information bias. • Difficult to validate information. • Control of extraneous variables may be incomplete. • Selection of an appropriate comparison group may be difficult. • Rates of disease in exposed and unexposed individuals cannot be determined.

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