What is the difference between cases and controls

Date last modified: June 7, Wayne W. Case-Control Studies. Sources of Cases Typical sources for cases include: Patient rosters at medical facilities Death certificates Disease registries e. In view of this there are two key principles that should be followed in selecting controls: The comparison group "controls" should be representative of the source population that produced the cases. The "controls" must be sampled in a way that is independent of the exposure, meaning that their selection should not be more or less likely if they have the exposure of interest.

Therefore, we would expect that about 1, would be exposed and 5, unexposed the same ratio as in the whole population , and came up with an odds ratio that was same as the hypothetical risk ratio we would have had if we had collected exposure information from the whole population of six million: What if we had instead been more likely to sample those who were exposed, so that we instead found 1, exposed and 4, unexposed among the 6, controls?

Then the odds ratio would have been: This odds ratio is biased because it differs from the true odds ratio. Example: A hypothetical case-control study was conducted to determine whether lower socioeconomic status the exposure is associated with a higher risk of cervical cancer the outcome.

Observed Assignment. The method of assignment of individuals to study and control groups in observational studies when the investigator does not intervene to perform the assignment. Unmatched Design. The controls are a sample from a suitable non-affected population.

Case Control Studies are prospective in that they follow the cases and controls over time and observe what occurs. Ask us. Now test yourself! Which of the following is an advantage of Case Control Studies?

Wow its amazing n simple way of briefing ,which i was enjoyed to learn this. Thanks n stay connected. It really helped me to understand the topic.

I am taking epidemiology class this winter, and your paper really saved me. Thank you for the easy to understand blog in cohort studies. As, in cohort studies we are looking at incidence new cases, so if an outcome have occurred before the exposure, I can leave them out of the analysis. But because I am not looking at a single outcome which can be checked easily and if happened before exposure can be left out.

I have EHR data, so all the exposure and outcome have occurred. Conducting successful research requires choosing the appropriate study design. This article describes the most common types of designs conducted by researchers.

A well-designed cohort study can provide powerful results. This blog introduces prospective and retrospective cohort studies, discussing the advantages, disadvantages and use of these type of study designs. In this blog, Leonardo provides 5 interpretations that you should consider when you read or hear about a reported association in observational studies.

What are the key steps in EBM? Who are S4BE? Case-control study cohort study outcome study design. View more posts from Saul. Leave a Reply Cancel reply Your email address will not be published. Hari Very much helpful 15th August at am Reply to Hari. Divine thanks for the information and knowledge about observational studies.

Marie L I have recently completed an investigational study where evidence of phlebitis was determined in a control cohort by data mining from electronic medical records. AJ So clear and perfect. Emma Carter Great to hear, thank you AJ! Vendelino Raymond Apreciated the information provided above. The odds ratio is a measure of the odds of disease in the exposed compared to the odds of disease in the unexposed controls and is calculated as:.

Example: Calculation of the OR from a hypothetical case-control study of smoking and cancer of the pancreas among cases and controls. Table 1. Hypothetical case-control study of smoking and cancer of the pancreas. NB: The odds ratio of smoking and cancer of the pancreas has been performed without adjusting for potential confounders.

Further analysis of the data would involve stratifying by levels of potential confounders such as age. The 2x2 table can then be extended to allow for stratum specific rates of the confounding variable s to be calculated and, where appropriate, an overall summary measure, adjusted for the effects of confounding, and a statistical test of significance can also be calculated. In addition, confidence intervals for the odds ratio would also be presented. References 1. Skip to main content.

Create new account Request new password. You are here Epidemiology for Practitioners. Introduction Learning objectives: You will learn about basic introduction to case-control studies, its analysis and interpretation of outcomes. Resource text Case-control studies start with the identification of a group of cases individuals with a particular health outcome in a given population and a group of controls individuals without the health outcome to be included in the study.

Issues in the design of case-control studies Formulation of a clearly defined hypothesis As with all epidemiological investigations the beginning of a case-control study should begin with the formulation of a clearly defined hypothesis. Selection of cases Case-control studies may use incident or prevalent cases.

These include: Standardized questionnaires Biological samples Interviews with the subject Interviews with spouse or other family members Medical records Employment records Pharmacy records The procedures used for the collection of exposure data should be the same for cases and controls. Common sources of bias in case-control studies Due to the retrospective nature of case-control studies, they are particularly susceptible to the effects of bias, which may be introduced as a result of a poor study design or during the collection of exposure and outcome data.

Analysis of case-control studies The odds ratio OR is used in case-control studies to estimate the strength of the association between exposure and outcome. The results of a case-control study can be presented in a 2x2 table as follow: The odds ratio is a measure of the odds of disease in the exposed compared to the odds of disease in the unexposed controls and is calculated as: Example: Calculation of the OR from a hypothetical case-control study of smoking and cancer of the pancreas among cases and controls.

Strengths and weaknesses of case-control studies Strengths Cost effective relative to other analytical studies such as cohort studies. Case-control studies are retrospective, and cases are identified at the beginning of the study; therefore there is no long follow up period as compared to cohort studies.

Efficient for the study of diseases with long latency periods. Efficient for the study of rare diseases. Good for examining multiple exposures. Weaknesses Particularly prone to bias; especially selection, recall and observer bias.