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Advantages of case control study vs cohort

Abstract This article discusses the observational analytic study designs, i. These two study designs are useful for testing a hypothesis to determine the association between a risk factor and a disease.

  • If the hypothesis is true, there should be a greater proportion of subjects in cells 'a' and 'd';
  • J Health Spec 2016;4:

The case—control studies start from the disease status and compare the exposure to the risk factor s between the diseased cases and the not diseased controls groups. The odds ratio is determined to compare the proportion of exposed persons in the two groups.

The cohort studies start from the exposure to the risk factor status and compare the incidence of the disease in the exposed and not exposed groups. The relative risk compares the incidence between the two groups.

The strengths and limitations of the two study designs differ based on the direction of the two designs. The case—control study goes backward from the disease status so is more useful for rare diseases and for evaluating multiple risk factors, but it cannot determine causality, and there are chances of recall bias affecting the results of the study.

The cohort studies are generally prospective in design from the exposure status and can determine the causal association between the risk factor and the disease. However, the cohort studies are more expensive and require a longer time as well as a larger sample size; the loss to follow-up and misclassification biases can affect the results of the cohort studies.

Selecting the appropriate study design: Case—control and cohort study designs.

J Health Spec 2016;4: The first two study designs are part of the 'observational' group of study designs along with the rest of the descriptive studies. Clinical trials are classified as 'interventional studies', which are also called as 'experimental' study designs.

However, the associations identified in descriptive studies cannot be definitively attributed to the identified risk factor s as there is no comparison group. Similarly, cell 'c' includes the subjects that are not exposed but diseased, while cell 'd' is the group that is not exposed and not diseased.

If the hypothesis is true, there should be a greater proportion of subjects in cells 'a' and 'd'. At the same time, there will always be subjects who are exposed to the risk factor but not diseased cell 'b' as well as those that are not exposed to the risk factor but are diseased cell 'c'.

Similarly, there are also many non-smokers who have heart disease cell 'c' but the majority of the non-smokers are not diseased cell 'd'. The explanation for this is that most of the non-communicable diseases are multifactorial in nature while even for the communicable diseases, exposure to the organism does not necessarily lead to the development of the disease.

The direction of the study is from the disease status to the exposure status [Table 3] and is useful for determining the risk factors that are associated with a disease.

  • His loss will be a difficult one to compensate in the field of medical education and research;
  • The direction of the study is from the disease status to the exposure status [Table 3] and is useful for determining the risk factors that are associated with a disease;
  • However, the cohort studies are more expensive and require a longer time as well as a larger sample size; the loss to follow-up and misclassification biases can affect the results of the cohort studies.

The two groups are compared with regards to the proportion of exposure to the risk factors s of interest in each group. The direction and evaluation of a case-control study design Click here to view The OR is used for comparing the proportion of exposure between the two groups of cases and controls. The OR is determined by comparing the ratio of exposed with the not exposed in the diseased group with the ratio of exposed with the not exposed in the controls.

This is given by the following formula: The greater the difference in the exposure between the two groups, the higher the value of the OR. If both the CI values are above '1', it indicates that there is a positive association between the disease and the risk factor. This study design can be used to test for several risk factors for a single disease and is especially useful for rare diseases.

It requires less time and is less expensive to conduct as compared to the other analytical study designs. The main disadvantage of the case—control study designs is that they cannot determine if the risk factor causes the disease since it cannot be determined whether most of the risk factors occurred before the disease. This is important to consider when interpreting the results of case—control studies that should not be stated as to imply that there is causative relationship between the risk factor and the disease, e.

  1. Strengths and limitations of the case-control and cohort study designs Click here to view Acknowledgment This article is the fourth one in the series on articles on Research Methods. It requires less time and is less expensive to conduct as compared to the other analytical study designs.
  2. It was clear that the exposure preceded adverse outcomes among exposed subjects who developed problems.
  3. It is likely that people who are diseased cases remember their exposure to the risk factor more accurately as compared to the controls. The relative risk compares the incidence between the two groups.

Instead, the appropriate statement for a case—control study should be that 'there is an association between the disease and the risk factor' or 'cases are 2 times more likely to be exposed to the risk factor as compared to controls'. In this case, the difference in the exposure may be due to some other inherent differences between the groups rather than the risk factor being studied. The recall bias is most applicable to case—control studies where the subjects are asked to answer questions about exposure to risk factors in the past.

Cohort Studies

It is likely that people who are diseased cases remember their exposure to the risk factor more accurately as compared to the controls. This may result in the OR value being higher than the actual value. It is important to note that case—control study designs cannot determine neither the prevalence or incidence of the disease nor the risk factors as the subjects are generally collected by purposive sampling.

If the number of cases is sufficient, a group of controls can be selected who are matched for certain criteria such as gender, age and group which are not part of the variables being considered risk factors in the study. This makes the case—control study a practical and convenient study design to be applied in hospital or clinic settings where both diseased cases and non-diseased controls can be easily accessed.

The two groups are longitudinally followed-up over time to observe the occurrence of the outcome of interest in each group. The direction of the study is from the exposure status to the outcome status [Table 4] and is useful for comparing the incidence of disease in the two groups. The direction and evaluation of a cohort study design Click here to view The RR is the epidemiological measure of association that is applied for the analysis of the results in cohort studies.

If the incidence in the two groups is equal, the value for RR will be '1' but if the value is greater than '1', this indicates a positive 'causal' relationship between the risk factor and the disease. The main advantage is that all the subjects are disease-free at the beginning of the study, so causality of the risk factor can be determined since the exposure precedes the outcome.

This requires a relatively larger sample size advantages of case control study vs cohort upon the incidence rate of the outcome and also the expected loss to follow-up rate due to subjects dropping out from any or both of the groups.

The cohort study is not suitable for studying rare diseases or outcomes since this will require a very large sample size to get sufficient number of outcomes for analysing the data. However, for certain exposures such as blood group, genetic markers or other factors that clearly occurred earlier, it may be possible for conducting retrospective cohort studies.

The exposure status is established in the past from medical records or medical history, and the outcome status is determined at the time of the study and after follow-up for a period if required.

  • This makes the case—control study a practical and convenient study design to be applied in hospital or clinic settings where both diseased cases and non-diseased controls can be easily accessed;
  • It is likely that people who are diseased cases remember their exposure to the risk factor more accurately as compared to the controls.

The biases that may affect the results of the cohort studies include loss to follow-up bias, especially if the loss to follow-up is more in one group as compared to the other group. The other bias is related to the selection bias - the two groups must be comparable to each other except for the exposure status. This may also be due to the fact that during the follow-up period, the exposure status of the subjects may change leading to inappropriate analysis of the results.

However, the two studies are methodologically different in that the case—control study starts from the outcome and goes 'back' to determine the exposure to the risk factor, while the cohort study starts from the exposure status and goes 'forward' to determine the incidence of outcome in the groups to be compared. In this way, the two study designs are more suitable for different types of outcomes and risk factors, and each one has its own strengths and limitations as shown in [Table 5].

Both study designs are observational studies, so the chance of confounding due to factors inherent to the group classification is still present. However, these two still constitute the most common study designs that are used in the epidemiological field along with the cross-sectional studies and the clinical trials.

  1. The cohort study is not suitable for studying rare diseases or outcomes since this will require a very large sample size to get sufficient number of outcomes for analysing the data.
  2. However, the two studies are methodologically different in that the case—control study starts from the outcome and goes 'back' to determine the exposure to the risk factor, while the cohort study starts from the exposure status and goes 'forward' to determine the incidence of outcome in the groups to be compared.
  3. It requires less time and is less expensive to conduct as compared to the other analytical study designs. The OR is determined by comparing the ratio of exposed with the not exposed in the diseased group with the ratio of exposed with the not exposed in the controls.
  4. The direction and evaluation of a case-control study design Click here to view The OR is used for comparing the proportion of exposure between the two groups of cases and controls.
  5. The case—control study goes backward from the disease status so is more useful for rare diseases and for evaluating multiple risk factors, but it cannot determine causality, and there are chances of recall bias affecting the results of the study. The first two study designs are part of the 'observational' group of study designs along with the rest of the descriptive studies.

Strengths and limitations of the case-control and cohort study designs Click here to view Acknowledgment This article is the fourth one in the series on articles on Research Methods.

I would like to dedicate this article series to Professor James Ware, who was the main motivator behind these articles. His continuous support and valuable feedback on each article were instrumental in improving the quality of the articles. Professor James Ware's lively personality will be dearly missed by everyone who had the opportunity of working with him.

He had an exuberance which he transferred to the people around him. His loss will be a difficult one to compensate in the field of medical education and research. We should strive to carry on his work in the same manner by providing support and guidance to anyone that needs assistance in the things that we learned with Professor James Ware.

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